feat(annotations): Figma-style UI overhaul

- Redesign PinOverlay: teardrop pins with author initials, colors, numbers
- Split FeedbackPanel into modular components:
  - feedback/FeedbackPanel.tsx (orchestrator)
  - feedback/ThreadList.tsx (list view with filters)
  - feedback/ThreadListItem.tsx (individual thread row)
  - feedback/ThreadDetail.tsx (expanded thread with comments)
  - feedback/CommentItem.tsx (single comment)
  - feedback/CommentInput.tsx (reusable input with Enter/Shift+Enter)
  - feedback/feedbackStyles.ts (shared design tokens)
- Add status colors: red (open), green (resolved)
- Add relative timestamps (2h ago, 3d, etc.)
- Wire pin clicks to expand threads in panel
- Pins follow media transforms in real-time
- Add delete thread with inline confirmation
- Register review mode shortcut (. key)
- Better empty states and filter bar (Open/Resolved/All/Mine)

frontend/src/components/Toolbar.tsx

@@ -315,7 +315,7 @@ export default function Toolbar({
 
       {/* Review / Feedback */}
       {onToggleReview && (
-        <ActionBtn onClick={onToggleReview} title="Review Mode (R)" active={reviewMode}>
+        <ActionBtn onClick={onToggleReview} title="Review Mode (.)" active={reviewMode}>
           <svg width="14" height="14" viewBox="0 0 14 14" fill="none" stroke="currentColor" strokeWidth="1.3">
             <path d="M2 2.5A1.5 1.5 0 013.5 1h7A1.5 1.5 0 0112 2.5v6A1.5 1.5 0 0110.5 10H6l-3 3v-3H3.5A1.5 1.5 0 012 8.5v-6z" />
           </svg>