fix(annotations): address code review findings (1-8, 10)

1. toggleVote wrapped in transaction (race condition fix)
2. FeedbackPanel fetch calls now surface errors via onError/toast
3. Extracted resolveBoard/hasCollectionRole to shared board-access.js
4. AnnotationStore uses monotonic version counter for snapshots
5. PinOverlay uses object pool instead of destroy/recreate on refresh
6. canvasObjects prop memoized with useMemo
7. PinOverlay store subscription cleaned up on unmount
8. Comment content capped at 5000 chars (backend validation)
10. anchor_type validated to 'object' or 'point'

frontend/src/stores/annotationStore.ts

@@ -42,6 +42,10 @@ export class AnnotationStore {
   votes = new Map<string, Set<string>>();
 
   private _listeners = new Set<Listener>();
+  private _version = 0;
+
+  /** Monotonic version counter for useSyncExternalStore snapshots */
+  get version(): number { return this._version; }
 
   subscribe(fn: Listener): () => void {
     this._listeners.add(fn);
@@ -49,6 +53,7 @@ export class AnnotationStore {
   }
 
   private _notify() {
+    this._version++;
     for (const fn of this._listeners) fn();
   }