feat(refboard): canvas polish — transform box sync, dot grid, snap tuning, perf fixes

- Fix transform box not updating after alignment/arrangement/normalize/flip operations
  (shortcuts, context menu, and selection toolbar all fixed with DRY _opUpdate helper)
- Replace 100ms polling loop with event-driven viewport updates (moved + wheel-scroll)
- Add adaptive dot grid background that responds to zoom/pan
- Reduce snap guide threshold from 8px to 4px for subtler snapping
- Remove PresenceOverlay (remote selection highlighting) — too heavy for minimal benefit
- Offset multiple dropped images so they don't overlap
- Add new canvas modules: SnapGuides, clipboard, grouping, FrameSprite, DrawingSprite,
  LaserPointer, context-menu-items, SelectionToolbar, Minimap
- Extract Editor hooks into dedicated files (useBoardLoader, useCanvasSetup,
  useShortcutHandler, useLayerPanel, useSaveManager, useFollowMode)
- Sync improvements: real-time transform broadcast, board rooms, viewport sync

frontend/src/hooks/useBoardLoader.ts

@@ -0,0 +1,44 @@
+import { useEffect, useState } from 'react';
+import { getBoard } from '../api';
+
+interface BoardLoaderResult {
+  boardData: any;
+  loading: boolean;
+  error: string;
+}
+
+/**
+ * Loads board data by ID with cancellation support.
+ */
+export function useBoardLoader(boardId: string | undefined): BoardLoaderResult {
+  const [boardData, setBoardData] = useState<any>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState('');
+
+  useEffect(() => {
+    let cancelled = false;
+    async function load() {
+      try {
+        if (!boardId) {
+          setError('No board specified');
+          setLoading(false);
+          return;
+        }
+        const res = await getBoard(boardId);
+        if (!cancelled) {
+          setBoardData(res.data);
+          setLoading(false);
+        }
+      } catch (err: any) {
+        if (!cancelled) {
+          setError(err.response?.data?.error || 'Failed to load board');
+          setLoading(false);
+        }
+      }
+    }
+    load();
+    return () => { cancelled = true; };
+  }, [boardId]);
+
+  return { boardData, loading, error };
+}