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/App.tsx

@@ -197,8 +197,12 @@ function AppShell() {
       && (orbitPhase === 'active' || orbitPhase === 'joining' || orbitPhase === 'starting');
     if (inOrbit) {
       document.documentElement.setAttribute('data-orbit-active', 'true');
+      // Also expose the role so CSS can target host-vs-guest UI states
+      // (e.g. guest seekbar is read-only — sync follows the host).
+      document.documentElement.setAttribute('data-orbit-role', orbitRole as string);
     } else {
       document.documentElement.removeAttribute('data-orbit-active');
+      document.documentElement.removeAttribute('data-orbit-role');
     }
   }, [orbitRole, orbitPhase]);