perf(storage): offload file pruning to background thread

Moves the synchronous file I/O logic in `run_backups` to a background thread (`std:🧵:spawn`) so that deleting old snapshots or deleted-sheets files does not block the main thread execution.

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

src-tauri/src/main.rs

@@ -477,6 +477,8 @@ fn run_backups(app: AppHandle, dir: Option<String>, retention_days: u32) -> Resu
         })
         .collect();
     snaps.sort();
+
+    std::thread::spawn(move || {
         if snaps.len() > SNAPSHOT_KEEP {
             for old in &snaps[..snaps.len() - SNAPSHOT_KEEP] {
                 fs::remove_file(old).ok();
@@ -495,6 +497,8 @@ fn run_backups(app: AppHandle, dir: Option<String>, retention_days: u32) -> Resu
                 }
             }
         }
+    });
+
     Ok(())
 }