fix(orbit): event-driven host push + guest seekbar lock (#537)

* fix(orbit): event-driven host push on play/pause flips

Without this, the worst-case delay between "host hits pause" and "guest
stops" is two full polling windows (host's 2.5 s push tick + guest's
2.5 s read tick, plus network) — long enough for the guest to noticeably
run past the host. Subscribing to playerStore.isPlaying changes adds at
most one extra remote write per flip; non-flip state ticks still ride
the existing 2.5 s timer. The listener filters on isPlaying so the
per-second currentTime ticks don't trigger spurious pushes.

* fix(orbit): lock seekbar for guests — sync follows the host

Guests could drag/click/wheel the seekbar, which would jump the local
player and then snap back at the next host poll (2.5 s of inconsistent
UX) — or push the guest into a diverged state where Catch Up was the
only way back. The seekbar is host-controlled in Orbit; the guest input
path now reflects that.

- App.tsx exposes `data-orbit-role="host"|"guest"` on the root element
  alongside the existing `data-orbit-active` marker.
- WaveformSeek's container gains a `.waveform-seek-container` class so
  CSS can target it.
- Guest rule: `pointer-events: none` on children blocks click / drag /
  wheel / hover; the parent keeps `cursor: not-allowed` + reduced opacity
  so the disabled state is visually unambiguous.

Hosts and non-orbit users see no change.

* docs(changelog): credit PR #537 (orbit sync latency + guest seekbar)

src/hooks/useOrbitHost.ts

@@ -225,7 +225,26 @@ export function useOrbitHost(): void {
     void pushState();
 
     const id = window.setInterval(() => { void pushState(); }, STATE_TICK_MS);
-    return () => window.clearInterval(id);
+
+    // Event-driven push on play/pause flips. Without this the worst-case
+    // delay between "host hits pause" and "guest stops" is two full polling
+    // windows (host tick + guest tick = up to 5 s) — long enough for the
+    // guest to noticeably run ahead. Subscribing here adds at most one
+    // extra write per flip; non-flip state ticks still ride the 2.5 s
+    // timer. Listener filters on isPlaying so currentTime ticks don't
+    // trigger spurious pushes.
+    let prevIsPlaying = usePlayerStore.getState().isPlaying;
+    const unsubPlayPause = usePlayerStore.subscribe((state) => {
+      if (state.isPlaying === prevIsPlaying) return;
+      prevIsPlaying = state.isPlaying;
+      pushOrbitEvent('host:event-push', `isPlaying flip → ${state.isPlaying}`);
+      void pushState();
+    });
+
+    return () => {
+      window.clearInterval(id);
+      unsubPlayPause();
+    };
   }, [active, sessionPlaylistId]);
 
   useEffect(() => {