Realtime sync: implement leader-election for DB saves

[th3fallen]

Context

From PR #85 review — currently all connected clients independently auto-save the game to the database on a 60s debounce. With multiple users connected, this means redundant writes to Supabase.

Proposal

Implement a leader-election mechanism using Supabase's Realtime Presence API so that only the first/oldest connected client is responsible for saving to the database. This models the peer-to-peer connection as a coordinated primary/secondary election.

Considerations

• Use Supabase Presence to track connected peers and determine the "leader" (e.g., lowest SENDER_ID or first to join)
• Only the leader triggers saveRemoteGame on the auto-save debounce
• If the leader disconnects, a new leader is elected and takes over save responsibility
• Non-leader clients still broadcast patches via realtime — they just don't write to the DB
• Edge case: if the leader crashes without a clean disconnect, presence should eventually time out and trigger re-election

Related

• PR Add realtime sync #85 — realtime sync implementation
• src/games/sync/useRealtimeGameSync.ts — scheduleAutoSave() function