feat(annotations): comments, threads, voting system with review mode

Backend:
- comment_threads + comments + object_votes tables with indexes
- Thread/comment CRUD endpoints with socket broadcast
- Vote toggle endpoint with socket broadcast

Frontend:
- AnnotationStore for reactive thread/comment/vote state
- FeedbackPanel with thread list, expanded view, replies, filtering
- Vote toggle buttons in panel
- PinOverlay for canvas pin markers + vote badges
- Review Mode toggle in toolbar (comment bubble icon)
- Jump-to-object from thread view
- Orphaned thread detection for deleted objects
- New comment creation from panel when object selected
- Socket event wiring for real-time sync

frontend/src/components/Toolbar.tsx

@@ -34,6 +34,8 @@ interface ToolbarProps {
   onToggleHelp?: () => void;
   onMmImport?: () => void;
   onExport?: () => void;
+  onToggleReview?: () => void;
+  reviewMode?: boolean;
   boardName?: string;
 }
 
@@ -150,6 +152,8 @@ export default function Toolbar({
   onToggleHelp,
   onMmImport,
   onExport,
+  onToggleReview,
+  reviewMode,
 }: ToolbarProps) {
   const showStroke = activeTool === ToolType.PEN;
   const showFontSize = activeTool === ToolType.TEXT;
@@ -309,6 +313,15 @@ export default function Toolbar({
         </ActionBtn>
       )}
 
+      {/* Review / Feedback */}
+      {onToggleReview && (
+        <ActionBtn onClick={onToggleReview} title="Review Mode (R)" 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>
+        </ActionBtn>
+      )}
+
       {/* Spacer */}
       <div style={{ flex: 1 }} />