diff --git a/src/storage.test.ts b/src/storage.test.ts
index f43c6f7..12d45b1 100644
--- a/src/storage.test.ts
+++ b/src/storage.test.ts
@@ -1,11 +1,16 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
-import { isTauri, loadSettings, saveSettings, defaultSettingsData } from './storage';
+import { isTauri, loadSettings, saveSettings, defaultSettingsData, loadAppData, saveAppData, flushAppData, setDataDir, runBackups, backupDeletedSheet, openBackupsFolder, chooseFolder, dataDirHasDocuments, migrateDataDir, fetchRates, fetchHistoricalRates } from './storage';
 import { invoke } from '@tauri-apps/api/core';
+import { open } from '@tauri-apps/plugin-dialog';
 
 vi.mock('@tauri-apps/api/core', () => ({
   invoke: vi.fn(),
 }));
 
+vi.mock('@tauri-apps/plugin-dialog', () => ({
+  open: vi.fn(),
+}));
+
 describe('storage', () => {
   beforeEach(() => {
     vi.resetAllMocks();
@@ -15,6 +20,7 @@ describe('storage', () => {
       setItem: vi.fn(),
     });
     vi.stubGlobal('console', { warn: vi.fn() });
+    vi.stubGlobal('fetch', vi.fn());
   });
 
   afterEach(() => {
@@ -93,4 +99,323 @@ describe('storage', () => {
       expect(invoke).toHaveBeenCalledWith('save_file', { name: 'settings.json', contents: JSON.stringify(settings, null, 2) });
     });
   });
+
+  describe('loadAppData', () => {
+    const mockAppData = { docs: [], activeId: '1', contents: {} };
+
+    it('loads from localStorage when not in Tauri', async () => {
+      vi.mocked(localStorage.getItem).mockReturnValue(JSON.stringify(mockAppData));
+      const data = await loadAppData('');
+      expect(localStorage.getItem).toHaveBeenCalledWith('summarum.documents');
+      expect(data).toEqual(mockAppData);
+    });
+
+    it('loads via invoke when in Tauri', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      vi.mocked(invoke).mockResolvedValue(JSON.stringify(mockAppData));
+      const data = await loadAppData('');
+      expect(invoke).toHaveBeenCalledWith('load_documents', { dir: null });
+      expect(data).toEqual(mockAppData);
+    });
+
+    it('loads via invoke when in Tauri with specific dataDir', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      vi.mocked(invoke).mockResolvedValue(JSON.stringify(mockAppData));
+      const data = await loadAppData('custom_dir');
+      expect(invoke).toHaveBeenCalledWith('load_documents', { dir: 'custom_dir' });
+      expect(data).toEqual(mockAppData);
+    });
+
+    it('returns null if localStorage throws', async () => {
+      vi.mocked(localStorage.getItem).mockImplementation(() => { throw new Error('localStorage error'); });
+      const data = await loadAppData('');
+      expect(console.warn).toHaveBeenCalledWith('loadAppData failed', expect.any(Error));
+      expect(data).toBeNull();
+    });
+
+    it('returns null if invoke throws', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      vi.mocked(invoke).mockRejectedValue(new Error('invoke error'));
+      const data = await loadAppData('');
+      expect(console.warn).toHaveBeenCalledWith('loadAppData failed', expect.any(Error));
+      expect(data).toBeNull();
+    });
+
+    it('returns null if localStorage returns null', async () => {
+      vi.mocked(localStorage.getItem).mockReturnValue(null);
+      const data = await loadAppData('');
+      expect(data).toBeNull();
+    });
+
+    it('returns null if JSON is invalid', async () => {
+      vi.mocked(localStorage.getItem).mockReturnValue("invalid json");
+      const data = await loadAppData('');
+      expect(console.warn).toHaveBeenCalledWith('loadAppData failed', expect.any(Error));
+      expect(data).toBeNull();
+    });
+  });
+
+  describe('saveAppData & flushAppData', () => {
+    const mockAppData = { docs: [], activeId: '1', contents: {} };
+
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+
+    it('debounces saves and flushes correctly when not in Tauri', async () => {
+      setDataDir('');
+      saveAppData(mockAppData);
+
+      // Should not save immediately
+      expect(localStorage.setItem).not.toHaveBeenCalled();
+
+      // Fast forward past debounce time
+      vi.advanceTimersByTime(400);
+
+      expect(localStorage.setItem).toHaveBeenCalledWith('summarum.documents', JSON.stringify(mockAppData));
+    });
+
+    it('debounces saves and flushes correctly when in Tauri', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+
+      setDataDir('custom_dir');
+      saveAppData(mockAppData);
+
+      // Should not save immediately
+      expect(invoke).not.toHaveBeenCalled();
+
+      // Fast forward past debounce time
+      vi.runAllTimers();
+
+      // Allow dynamic import and promise resolution by temporarily using real timers
+      vi.useRealTimers();
+      await new Promise(resolve => setTimeout(resolve, 0));
+      vi.useFakeTimers();
+
+      expect(invoke).toHaveBeenCalledWith('save_documents', { dir: 'custom_dir', contents: JSON.stringify(mockAppData) });
+    });
+
+    it('flushAppData clears timer and saves immediately', async () => {
+      setDataDir('');
+      saveAppData(mockAppData);
+
+      await flushAppData();
+
+      expect(localStorage.setItem).toHaveBeenCalledWith('summarum.documents', JSON.stringify(mockAppData));
+
+      // Advancing timers should not trigger another save
+      vi.mocked(localStorage.setItem).mockClear();
+      vi.advanceTimersByTime(400);
+      expect(localStorage.setItem).not.toHaveBeenCalled();
+    });
+
+    it('flushAppData does nothing if no pending data', async () => {
+      await flushAppData();
+      expect(localStorage.setItem).not.toHaveBeenCalled();
+      expect(invoke).not.toHaveBeenCalled();
+    });
+
+    it('handles localStorage throw gracefully in flushAppData', async () => {
+      setDataDir('');
+      saveAppData(mockAppData);
+      vi.mocked(localStorage.setItem).mockImplementation(() => { throw new Error('localStorage error'); });
+
+      await flushAppData();
+
+      expect(console.warn).toHaveBeenCalledWith('saveAppData failed', expect.any(Error));
+    });
+
+    it('handles invoke throw gracefully in flushAppData', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      setDataDir('');
+      saveAppData(mockAppData);
+      vi.mocked(invoke).mockRejectedValue(new Error('invoke error'));
+
+      await flushAppData();
+
+      expect(console.warn).toHaveBeenCalledWith('saveAppData failed', expect.any(Error));
+    });
+  });
+
+  describe('backup and utility functions', () => {
+    describe('runBackups', () => {
+      it('does nothing when not in Tauri', async () => {
+        await runBackups('dir', 10);
+        expect(invoke).not.toHaveBeenCalled();
+      });
+
+      it('calls invoke when in Tauri', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        await runBackups('dir', 10);
+        expect(invoke).toHaveBeenCalledWith('run_backups', { dir: 'dir', retentionDays: 10 });
+      });
+
+      it('handles invoke throw gracefully', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        vi.mocked(invoke).mockRejectedValue(new Error('invoke error'));
+        await runBackups('dir', 10);
+        expect(console.warn).toHaveBeenCalledWith('runBackups failed', expect.any(Error));
+      });
+    });
+
+    describe('backupDeletedSheet', () => {
+      it('does nothing when not in Tauri', async () => {
+        await backupDeletedSheet('dir', 'title', 'contents');
+        expect(invoke).not.toHaveBeenCalled();
+      });
+
+      it('does nothing if contents are empty', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        await backupDeletedSheet('dir', 'title', '   ');
+        expect(invoke).not.toHaveBeenCalled();
+      });
+
+      it('calls invoke when in Tauri with valid contents', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        await backupDeletedSheet('dir', 'title', 'contents');
+        expect(invoke).toHaveBeenCalledWith('backup_deleted_sheet', { dir: 'dir', title: 'title', contents: 'contents' });
+      });
+
+      it('handles invoke throw gracefully', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        vi.mocked(invoke).mockRejectedValue(new Error('invoke error'));
+        await backupDeletedSheet('dir', 'title', 'contents');
+        expect(console.warn).toHaveBeenCalledWith('backupDeletedSheet failed', expect.any(Error));
+      });
+    });
+
+    describe('openBackupsFolder', () => {
+      it('does nothing when not in Tauri', async () => {
+        await openBackupsFolder('dir');
+        expect(invoke).not.toHaveBeenCalled();
+      });
+
+      it('calls invoke when in Tauri', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        await openBackupsFolder('dir');
+        expect(invoke).toHaveBeenCalledWith('open_backups_folder', { dir: 'dir' });
+      });
+    });
+
+    describe('chooseFolder', () => {
+      it('returns null when not in Tauri', async () => {
+        expect(await chooseFolder()).toBeNull();
+      });
+
+      it('returns path when in Tauri and folder is chosen', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        vi.mocked(open).mockResolvedValue('chosen_path');
+        expect(await chooseFolder()).toBe('chosen_path');
+        expect(open).toHaveBeenCalledWith({ directory: true, multiple: false });
+      });
+
+      it('returns null when in Tauri and folder choice is cancelled', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        vi.mocked(open).mockResolvedValue(null);
+        expect(await chooseFolder()).toBeNull();
+      });
+    });
+
+    describe('dataDirHasDocuments', () => {
+      it('returns false when not in Tauri', async () => {
+        expect(await dataDirHasDocuments('dir')).toBe(false);
+      });
+
+      it('calls invoke when in Tauri', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        vi.mocked(invoke).mockResolvedValue(true);
+        expect(await dataDirHasDocuments('dir')).toBe(true);
+        expect(invoke).toHaveBeenCalledWith('data_dir_has_documents', { dir: 'dir' });
+      });
+    });
+
+    describe('migrateDataDir', () => {
+      it('does nothing when not in Tauri', async () => {
+        await migrateDataDir('old', 'new', 'move');
+        expect(invoke).not.toHaveBeenCalled();
+      });
+
+      it('calls invoke when in Tauri', async () => {
+        vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+        await migrateDataDir('old', 'new', 'move');
+        expect(invoke).toHaveBeenCalledWith('migrate_data_dir', { oldDir: 'old', newDir: 'new', strategy: 'move' });
+      });
+    });
+  });
+
+  describe('fetchRates', () => {
+    it('uses invoke when in Tauri', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      const mockRates = { date: '2024-01-01', rates: { EUR: 0.85 }, fetchedAt: 12345 };
+      vi.mocked(invoke).mockResolvedValue(mockRates);
+
+      const result = await fetchRates(true);
+      expect(invoke).toHaveBeenCalledWith('fetch_rates', { force: true });
+      expect(result).toEqual(mockRates);
+    });
+
+    it('uses fetch when not in Tauri', async () => {
+      const mockResponse = { result: 'success', time_last_update_utc: '2024-01-01', rates: { EUR: 0.85 } };
+      vi.mocked(fetch).mockResolvedValue({ json: vi.fn().mockResolvedValue(mockResponse) } as unknown as Response);
+
+      const result = await fetchRates();
+      expect(fetch).toHaveBeenCalledWith('https://open.er-api.com/v6/latest/USD');
+      expect(result?.date).toBe('2024-01-01');
+      expect(result?.rates).toEqual({ EUR: 0.85 });
+      expect(result?.fetchedAt).toBeTypeOf('number');
+    });
+
+    it('returns null if fetch fails', async () => {
+      vi.mocked(fetch).mockRejectedValue(new Error('Network error'));
+      const result = await fetchRates();
+      expect(console.warn).toHaveBeenCalledWith('fetchRates failed', expect.any(Error));
+      expect(result).toBeNull();
+    });
+
+    it('returns null if API result is not success', async () => {
+      const mockResponse = { result: 'error', 'error-type': 'unsupported-code' };
+      vi.mocked(fetch).mockResolvedValue({ json: vi.fn().mockResolvedValue(mockResponse) } as unknown as Response);
+      const result = await fetchRates();
+      expect(result).toBeNull();
+    });
+  });
+
+  describe('fetchHistoricalRates', () => {
+    it('uses invoke when in Tauri', async () => {
+      vi.stubGlobal('window', { __TAURI_INTERNALS__: {} });
+      const mockRates = { EUR: 0.85 };
+      vi.mocked(invoke).mockResolvedValue(mockRates);
+
+      const result = await fetchHistoricalRates('2024-01-01');
+      expect(invoke).toHaveBeenCalledWith('fetch_historical_rates', { date: '2024-01-01' });
+      expect(result).toEqual(mockRates);
+    });
+
+    it('uses fetch when not in Tauri and formats rates correctly', async () => {
+      const mockResponse = { rates: { EUR: 0.85, GBP: 0.75, JPY: -1 } }; // negative value should be ignored
+      vi.mocked(fetch).mockResolvedValue({ json: vi.fn().mockResolvedValue(mockResponse) } as unknown as Response);
+
+      const result = await fetchHistoricalRates('2024-01-01');
+      expect(fetch).toHaveBeenCalledWith('https://api.frankfurter.dev/v1/2024-01-01?from=USD');
+      expect(result).toEqual({ USD: 1, EUR: 0.85, GBP: 0.75 }); // USD: 1 added, negative ignored
+    });
+
+    it('returns null if fetch fails', async () => {
+      vi.mocked(fetch).mockRejectedValue(new Error('Network error'));
+      const result = await fetchHistoricalRates('2024-01-01');
+      expect(console.warn).toHaveBeenCalledWith('fetchHistoricalRates failed', '2024-01-01', expect.any(Error));
+      expect(result).toBeNull();
+    });
+
+    it('returns null if rates missing in response', async () => {
+      const mockResponse = { amount: 1, base: 'USD', date: '2024-01-01' }; // missing rates
+      vi.mocked(fetch).mockResolvedValue({ json: vi.fn().mockResolvedValue(mockResponse) } as unknown as Response);
+      const result = await fetchHistoricalRates('2024-01-01');
+      expect(result).toBeNull();
+    });
+  });
 });