fix(call): clear soundboard safety timer + detach permission onchange

- CallSoundboard: the 30s safety timeout (which unsticks the one-at-a-time
  playingKey guard if audio never signals end) was never cleared, so it fired
  ~30s after every clip. It's now stored in a per-play token that done() clears
  by identity — a natural 'ended' cancels it, and a stale done() from a prior
  clip can't disarm a newer clip's timer (which matters because a rejected
  audio.play() fires neither ended nor error, leaving the timer as the only
  guard-reset). The unmount effect also clears any pending timer, and the timer
  is armed only when there's an audio element.

- PrescreenControls: useMediaPermissions set PermissionStatus.onchange but never
  removed it → a permission change after unmount setState'd a dead component and
  retained the callback. Now guards all setState with a cancelled flag and
  detaches onchange in the effect cleanup.

Bug-hunt findings from LOTUS_TODO. Three review passes (the last prescribed the
per-play token to close a shared-ref cross-play edge). Gate-green (tsc, eslint,
prettier, 922 tests, build).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/call/PrescreenControls.tsx

@@ -17,15 +17,29 @@ function useMediaPermissions(): MediaPermState {
   useEffect(() => {
     if (!navigator.permissions) {
       setState('unknown');
-      return;
+      return undefined;
     }
+    let cancelled = false;
+    let permStatus: PermissionStatus | undefined;
     navigator.permissions
       .query({ name: 'microphone' as unknown as PermissionDescriptor['name'] })
       .then((result) => {
+        if (cancelled) return;
+        permStatus = result;
         setState(result.state as MediaPermState);
-        result.onchange = () => setState(result.state as MediaPermState);
+        result.onchange = () => {
+          if (!cancelled) setState(result.state as MediaPermState);
+        };
       })
-      .catch(() => setState('unknown'));
+      .catch(() => {
+        if (!cancelled) setState('unknown');
+      });
+    // Detach the onchange handler on unmount so it can't setState afterward (and
+    // so the PermissionStatus doesn't retain the callback).
+    return () => {
+      cancelled = true;
+      if (permStatus) permStatus.onchange = null;
+    };
   }, []);
 
   return state;