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/utils/relativeTime.ts

@@ -0,0 +1,11 @@
+export function relativeTime(iso: string): string {
+  const diff = Date.now() - new Date(iso).getTime();
+  const mins = Math.floor(diff / 60000);
+  if (mins < 1) return 'now';
+  if (mins < 60) return `${mins}m`;
+  const hrs = Math.floor(mins / 60);
+  if (hrs < 24) return `${hrs}h`;
+  const days = Math.floor(hrs / 24);
+  if (days < 7) return `${days}d`;
+  return new Date(iso).toLocaleDateString();
+}