fix(orbit): dedupe guest outbox entries per host sweep

Guest outboxes are append-only from the host's POV — every sweep reads
the same playlist. `applyOutboxSnapshotsToState` was unconditionally
pushing every trackId in every snapshot into `state.queue`, so the
pending-approval list grew by one duplicate per tick for every unhandled
suggestion (visible as 4+ identical rows after ~10 s).

Dedupe against `(user, trackId)` already present in `state.queue` or
`state.currentTrack` before appending. A guest re-suggesting the same
track after it lands/plays is a rare enough case to live with for now.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/utils/orbit.ts

@@ -870,9 +870,24 @@ export function applyOutboxSnapshotsToState(
   nowMs: number = Date.now(),
 ): OrbitState {
   // ── Queue additions ──
+  // Guest outboxes are append-only from the host's POV — the host reads the
+  // same playlist every sweep, so we must dedupe against anything already in
+  // `state.queue` (or currently playing) by (user, trackId). Without this,
+  // every host tick re-adds every outbox entry and the pending-approval list
+  // balloons indefinitely. A user re-suggesting the same track after it
+  // lands/plays is a rare enough case to live with for now.
+  const existingKeys = new Set<string>(
+    state.queue.map(q => `${q.addedBy}${q.trackId}`),
+  );
+  if (state.currentTrack) {
+    existingKeys.add(`${state.currentTrack.addedBy}${state.currentTrack.trackId}`);
+  }
   const newItems: OrbitQueueItem[] = [];
   for (const snap of snapshots) {
     for (const trackId of snap.trackIds) {
+      const key = `${snap.user}${trackId}`;
+      if (existingKeys.has(key)) continue;
+      existingKeys.add(key);
       newItems.push({ trackId, addedBy: snap.user, addedAt: nowMs });
     }
   }