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/socket/viewport-sync.js

@@ -0,0 +1,48 @@
+/**
+ * Viewport sync relay — broadcasts user viewport positions for follow mode.
+ * Each user periodically emits their viewport state; this relays it to
+ * everyone else in the same board room so followers can track in real-time.
+ *
+ * Kept separate from board-room.js to avoid conflicts.
+ */
+
+function getRoomName(boardId) {
+  return `board:${boardId}`;
+}
+
+function setupViewportSync(io, socket) {
+  // Relay viewport position to other users in the same board room
+  socket.on('viewport:sync', (data) => {
+    if (!socket.currentBoardId) return;
+    const roomName = getRoomName(socket.currentBoardId);
+    socket.volatile.to(roomName).emit('viewport:sync', {
+      ...data,
+      userId: socket.userId,
+    });
+  });
+
+  // follow:start / follow:stop are informational — the server doesn't need
+  // to gate viewport:sync relay (all users broadcast always, clients filter).
+  // These events exist so the UI can show "X is following you" if desired.
+  socket.on('follow:start', (data) => {
+    if (!socket.currentBoardId) return;
+    const roomName = getRoomName(socket.currentBoardId);
+    socket.to(roomName).emit('follow:start', {
+      boardId: data.boardId,
+      followerId: socket.userId,
+      followerName: socket.userDisplayName,
+      targetUserId: data.targetUserId,
+    });
+  });
+
+  socket.on('follow:stop', (data) => {
+    if (!socket.currentBoardId) return;
+    const roomName = getRoomName(socket.currentBoardId);
+    socket.to(roomName).emit('follow:stop', {
+      boardId: data.boardId,
+      followerId: socket.userId,
+    });
+  });
+}
+
+module.exports = { setupViewportSync };