feat(file-viewer): COD-341 search the full workspace

[aakhter]

Summary

• Wire the existing File Viewer search input to the server-side file search endpoint with a 250 ms debounce and strict response validation.
• Scope tree and search state to the active session, hidden-file preference, and independent request epochs so stale responses cannot repaint the panel.
• Preserve cached-tree restoration, safe directory-result navigation, and consistent reset behavior across session changes and both panel hide paths.

Test plan

• npm test -- test/file-browser-search.test.ts test/file-browser-hidden.test.ts test/routes/file-search-mode.test.ts
• npm run typecheck
• npm run lint
• npm run format:check
• npm run check:lockfile
• npx changeset status --since upstream/master
• node --check for app.js, panels-ui.js, and settings-ui.js
• npm run build

UI

Uses the existing File Viewer search input and row styling; no markup or CSS changes.