fix(review): stabilize diff focus, navigation, and comments

[ahmedragab20]

Summary

• Upgrade @pierre/diffs from 1.3.2 to 1.4.1 and migrate editor factories/change events, annotation ownership, and saved-snapshot handling.
• Keep comment keyboard events local; guard page shortcuts against handled events, IME composition, editable/shadow-DOM origins, and dialogs.
• Stop passive sidebar tracking from stealing focus, remove idle viewport/search polling, and make line navigation bounded and cancellable.
• Preserve viewed state when jumping to a line; ordinary collapse no longer advances to another file.
• Compare actual diff content rather than hunk shape alone, and preserve unchanged renderer identities.
• Validate comment responses, upsert duplicate IDs, await persistence, and retain drafts/editors after failures.
• Add automated regression coverage. Remove the temporary manual browser fixture; production code and automated tests do not depend on it.

Scope

main (including #7) has been merged into this branch. PrReviewApp.tsx and usePrSession.ts match main; shared renderer/comment stability changes remain intact.

Verification

• pnpm test:ts: 160 files, 1,603 tests passed.
• pnpm build:ts: passed; client chunks over 500 kB still produce warnings.
• Final test-typing corrections: 32 targeted tests passed.
• Changed files have zero TypeScript errors. Repository-wide tsc --noEmit still reports 55 errors in unchanged files; it is not an all-green typecheck.
• Earlier manual browser checks using the temporary fixture (now removed): 200 rendered rows; textarea focus and scroll position preserved through diff refresh and remote comment insertion; Ctrl+Enter created one comment without opening a review dialog; failed submission preserved the body and showed an error; zero browser errors in those checks.

Before ready for merge

• Complete the broader manual cross-browser, edit-mode, and navigation matrix.
• Complete before/after latency measurements; no comparative performance baseline is claimed here.
• Human verification of the original scrolling and comment-submission workflows.

Automated verification passed again after fixture removal: 160 files, 1,603 tests. The manual verification items above remain outstanding.

Merge readiness update

• Merged main into the branch in 04aefa8; resolved conflicts without dropping feat(gh-pr): harden PR sessions and add conversation inbox #7’s PR session and conversation features.
• pnpm test: 167 Vitest files, 1,659 tests passed; 231 Rust tests passed.
• pnpm build:ts: passed with browser externalization and chunk-size warnings.
• GitHub CI is being verified before merge.
• The author explicitly waived the remaining manual verification checklist above for this merge. Those checks were not completed; no comparative latency baseline is claimed.