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/canvas/InboxZone.ts

@@ -141,8 +141,8 @@ export class InboxZone extends Container {
       sprite.width = sw;
       sprite.height = sh;
 
-      // Load thumbnail texture
-      this.textures.load(assetKey, 'thumb').then((tex) => {
+      // Load texture (GPU handles scaling)
+      this.textures.load(assetKey).then((tex) => {
         if (!sprite.destroyed) {
           sprite.texture = tex;
           sprite.width = sw;