fix: reconcile client state on websocket connect

[paolino]

Summary

• Adds a setSession websocket command carrying the client's full current tracks and definitions.
• Replaces server tracks/definitions from that payload while preserving server slot capacity, persists both stores, and broadcasts the new snapshot.
• Sends the current editor state on websocket open before connected playback transitions can emit activate/silence commands.

Root Cause

The served UI booted from the frontend seed while the backend session loaded from empty stores. Playback commands could therefore reference track IDs that existed in the client but not on the server, producing PlaybackTrackMissing.

Validation

• nix develop -c just unit Riptide.Protocol
• nix develop -c just unit Riptide.Server
• nix develop .#frontend --command just test
• ./gate.sh
• Served UI smoke with an empty temporary state directory: no Command failed or PlaybackTrackMissing in rendered DOM/server log.