Both were self-contained enough to live in their own modules —
src/ui/search.ts (Ctrl+F overlay, text and result-value queries) and
src/ui/export.ts (render the sheet + results to a PNG, clipboard or
file). Trims main.ts by about 150 lines.
Also fixes a real bug while it's in there: the PNG export's clipboard
fallback did btoa(String.fromCharCode(...bytes)), which blows the
call stack on large sheets (spreading a big array as call arguments).
Chunks the conversion in 0x8000-byte pieces instead.
