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

backend/minio.js

@@ -116,6 +116,8 @@ function getImageUrl(minioPath) {
   return `/api/images/${minioPath}`;
 }
 
+const MAX_FILE_SIZE = parseInt(process.env.MAX_FILE_SIZE_MB || '200', 10) * 1024 * 1024;
+
 module.exports = {
   minioClient,
   initBucket,
@@ -126,4 +128,5 @@ module.exports = {
   getImageUrl,
   MINIO_BUCKET,
   MIME_TO_EXT,
+  MAX_FILE_SIZE,
 };