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/styles/components.css

@@ -1915,6 +1915,18 @@
   display: none;
 }
 
+/* Guest in an Orbit session: seekbar is read-only — sync follows the host.
+   `pointer-events: none` blocks click / drag / wheel / hover; the dimmed
+   look + not-allowed cursor (set on the parent so it shows when hovered
+   over the now-inert child) signal "this control is not yours". */
+[data-orbit-role="guest"] .waveform-seek-container {
+  cursor: not-allowed;
+  opacity: 0.55;
+}
+[data-orbit-role="guest"] .waveform-seek-container > * {
+  pointer-events: none;
+}
+
 .playlist-suggestions .track-row:hover .playlist-suggestion-preview-btn,
 .playlist-suggestion-preview-btn:hover,
 .playlist-suggestion-preview-btn.is-previewing {