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/hooks/useShortcutHandler.ts

@@ -30,6 +30,7 @@ interface ShortcutHandlerDeps {
   setShowGrid: React.Dispatch<React.SetStateAction<boolean>>;
   setShowHelp: React.Dispatch<React.SetStateAction<boolean>>;
   setFocusMode: React.Dispatch<React.SetStateAction<boolean>>;
+  setReviewMode: React.Dispatch<React.SetStateAction<boolean>>;
 }
 
 /**
@@ -41,7 +42,7 @@ export function useShortcutHandler(deps: ShortcutHandlerDeps) {
     onCanvasChange, showToast, refreshLayers,
     handleGroup, handleUngroup,
     setActiveTool, setCanUndo, setCanRedo, setZoom,
-    setShowGrid, setShowHelp, setFocusMode,
+    setShowGrid, setShowHelp, setFocusMode, setReviewMode,
   } = deps;
 
   useEffect(() => {
@@ -89,6 +90,7 @@ export function useShortcutHandler(deps: ShortcutHandlerDeps) {
         toggleGrid: () => setShowGrid((v) => !v),
         toggleShowHelp: () => setShowHelp((v) => !v),
         toggleFocusMode: () => setFocusMode((v) => !v),
+        toggleReviewMode: () => setReviewMode((v) => !v),
         pasteFromSystemClipboard: async (): Promise<string> => {
           if (!resolvedBoardId) return 'No board';
           const msg = await pasteFromSystemClipboard(scene, viewport, resolvedBoardId, onCanvasChange);
@@ -148,6 +150,6 @@ export function useShortcutHandler(deps: ShortcutHandlerDeps) {
     onCanvasChange, showToast, refreshLayers,
     handleGroup, handleUngroup,
     setActiveTool, setCanUndo, setCanRedo, setZoom,
-    setShowGrid, setShowHelp, setFocusMode,
+    setShowGrid, setShowHelp, setFocusMode, setReviewMode,
   ]);
 }