fix(desktop): poll buffer when remote peers are connected

[johncarmack1984]

Summary

• The bufferSet event never reaches the iOS webview, so when a guest or another device changes the buffer via the ctl frame path, the lights update but the phone UI stays frozen
• Poll sync_buffer at 200 ms while useRemotePeers reports at least one connected peer or guest — a cheap mutex read matching the state echo's coalesce window
• Polling stops entirely when the desk is solo; TanStack Query pauses it when the app is backgrounded

Test plan

• Open a shared desk from a guest device, move a fader — verify the host phone's UI updates within ~200 ms
• Verify the host's desktop UI still updates instantly (event listener path unchanged)
• Close the guest desk / sign out — verify polling stops (no sync_buffer IPC calls in logs when solo)
• Verify fader drag on the host phone is still smooth (optimistic state, not affected by poll)