Add scoped find navigation to file previews

[redxzeta]

Problem

Long code and Markdown previews had no in-file navigation, while the application already owned Cmd/Ctrl+F for other focused surfaces.

Implementation

• add a preview-scoped Cmd+F on macOS / Ctrl+F elsewhere without stealing shortcuts from composer, browser, terminal, or other focused surfaces
• search already-loaded source contents client-side with deferred matching
• show total/current match counts, yellow match highlights, an orange active match, Enter/Shift+Enter navigation, and active-match scrolling
• reset query/index on close and recompute/clamp the active index when loaded file contents change
• share the compact accessible find controls with thread find and make the existing HTML highlight wrapper linear across text runs for large-file responsiveness

Architectural choices

• reuse the existing thread-find range, HTML-marking, and active-match primitives instead of adding a second highlighting engine
• keep shortcut ownership in a small preview-specific pure helper
• use the existing shared disclosure motion for the compact find panel
• preserve source/Markdown preview architecture; image, PDF, editable textarea, repository search, regex, replace, and OCR remain out of scope

Tests performed

• NODE_OPTIONS=--max-old-space-size=4096 bun run test in apps/web: 369 files / 4,287 tests passed
• focused logic tests: 20 passed
• real Chromium component tests for the new find flow: 4 passed
• real Chromium preview/thread regressions: 9 passed
• bun run fmt:check: passed
• bun run lint: 0 errors (existing workspace warnings remain)
• bun run typecheck: 7/7 packages passed
• NODE_OPTIONS=--max-old-space-size=4096 bun run build in apps/web: passed

Manual / browser validation

• exercised shortcut scope, click-to-focus, source highlighting, Markdown highlighting, match navigation, no-match feedback, close/reset focus restoration, and live query-cache file updates in Chromium
• started an isolated Forkara instance and confirmed both services plus HTTP 200; the shared T3 preview client could not navigate to the isolated port in this environment, so no full-app screenshot was captured

Known limitations

• intentionally searches only the currently loaded code/Markdown preview; it does not add repository search, replace, regex, or image/PDF OCR

Closes #153