fix(client): correctly handle coordinate SceneUI attachment

[AnrokX]

Summary

• Mark coordinate-based SceneUI instances for an initial position sync when they are created on the client.
• Clear any prior entity attachment when SceneUI.setPosition(...) is used, keeping attachment modes mutually exclusive.

Why This Is The Right Fix

• SceneUI can be attached to either an entity or a world position, not both.
• Without clearing attachedToEntityId on setPosition, update logic can continue following entity-path behavior.
• Without initial _needsUpdatePosition for coordinate-spawned SceneUI, the first world transform can be missed.

Testing

• Local manual validation on March 2, 2026:
  • Ran local server and local client, connected at http://localhost:5173/?join=local.hytopiahosting.com:8080&debug.
  • Verified SceneUI rendered and stayed hooked to the configured world coordinate.
• Temporary local-only test/workaround edits used during validation were reverted.
• This PR contains only client/src/ui/SceneUI.ts.

[AnrokX]

Edge case: while SceneUI is pending template registration, merge logic drops undefined fields. That can lose the detach signal (attachedToEntityId = undefined) when switching to position, so it may still instantiate in entity-attached mode.

Minimal fix: in pending merge, treat position and attachedToEntityId as mutually exclusive and clear the opposite field when one is set.

[AnrokX]

Closing as superseded by upstream PR: hytopiagg#8