Switch whiteboard collab to Excalidraw-style incremental scene sync (stop full-scene spam)

[hweihwang]

• Current state (ours):
  • Send: src/hooks/useSync.ts:186-211, 320-344 emits full SCENE_INIT every 500 ms from every
    client on any onChange (leading+trailing throttle), no diffing, no isDedicatedSyncer gate.
  • Receive: src/hooks/useCollaboration.ts:567-589 applies full element arrays; dedup is a JSON
    string compare scoped to the render, no versioning.
  • Echo: remote updateScene triggers onChange and re-sends the same scene.
  • Server: websocket_server/SocketManager.js:334 rebroadcasts blindly.
• Problem: High baseline traffic and CPU just by being in a room; identical scenes bounce;
  SCENE_INIT is misused for ongoing updates; no message version/id; elements always sent, only
  files are hashed.
• Main solution (follow Excalidraw collab pattern):
  • Upstream reference:
    • Incremental send only when scene version increases: broadcastElements in https://github.com/excalidraw/excalidraw/blob/master/excalidraw-app/collab/Collab.tsx#L900-L954
    • Throttled full-scene fallback: queueBroadcastAllElements just below in same file.
    • Receive merge after version filtering: handleRemoteSceneUpdate in https://github.com/excalidraw/excalidraw/blob/master/excalidraw-app/collab/Collab.tsx#L782-L820
  • Adopt that model:
    1. Track scene version (or reuse computeElementVersionHash) and broadcast only when it
       increases; add a throttled full-scene fallback.
    2. Split message types: keep SCENE_INIT for first sync, add SCENE_UPDATE for incremental;
       include message version/id so receivers drop stale or duplicate updates.
    3. Gate scene broadcasts to isDedicatedSyncer; others send cursors/viewport only.
    4. Add a receive-side last-version ref to skip reapply without stringifying payloads.
    5. Prevent echo: mark remote-applied scenes so onChange does not rebroadcast them.
    6. Files/images: add a lightweight dataURL hash map to skip unchanged file sends; keep on
       reliable channel.
• Impact: Much lower bandwidth and CPU per participant, fewer focus/interaction glitches for embeddables, clearer message semantics for future work.

[JKratto]

I believe this is related issue: #409