feat(35): scenes remember the room — BRB mutes the mic on entry

591 green, 0 warnings.

- `Scene.SceneMutesMic` / `SceneMutesDesktop` are **nullable booleans**: null = leave the mix alone, so
  a scene is passive until the creator says otherwise. Schema columns added, NULL for every existing
  layout except one deliberate default.
- **Applied once when the scene takes the PROGRAM output** (`TransitionToScene`, only when live or
  recording). Staging a scene offline is auditioning, not a stream edit — the test asserts that.
- **The creator is boss:** a manual mute survives any transition into a scene with no opinion; a scene
  that does have one wins on its next entry. `SetMicMuted`/`SetGameMuted` are idempotent so a repeated
  "mute" cannot clobber the level that unmute would restore.
- **BRB ships muted** (creator spec) for new layouts *and* via a one-time migration for existing rows
  where the creator has never expressed an opinion.
- UI: two three-state checkboxes under the layers list ("When this scene goes live"), styled with
  `PropertyPillCheck` so the theme test holds.

`SceneAudioTests` covers the whole rule in one integration test: default state, offline staging does
nothing, live entry mutes, manual unmute survives a passive scene, an opinionated scene wins, desktop
untouched, and all three states roundtrip through save/reload.

Needs the creator's eye: the checkboxes are new chrome in the left panel (indeterminate = passive).

ViewModels/MainViewModel.Audio.cs

@@ -294,6 +294,38 @@ public partial class MainViewModel
     /// lip-sync is right", so it deserves one click, not a drag.</summary>
     internal void SetSyncOffset(int milliseconds) => AudioSyncOffsetMs = milliseconds;
 
+    /// <summary>TASK 35: mute/unmute from a scene entry. Idempotent — a scene that says "mute" while
+    /// the mic is already muted must not overwrite the level it would restore.</summary>
+    internal void SetMicMuted(bool muted)
+    {
+        if (muted == MicMuted) return;
+        if (muted)
+        {
+            _volumeBeforeMute = MicVolume;
+            MicVolume = 0;
+        }
+        else
+        {
+            MicVolume = _volumeBeforeMute ?? 0.8;
+            BeginVolumeFlash();
+        }
+    }
+
+    /// <summary>TASK 35: as <see cref="SetMicMuted"/>, for desktop/game audio.</summary>
+    internal void SetGameMuted(bool muted)
+    {
+        if (muted == GameMuted) return;
+        if (muted)
+        {
+            _gameVolumeBeforeMute = GameAudioVolume;
+            GameAudioVolume = 0;
+        }
+        else
+        {
+            GameAudioVolume = _gameVolumeBeforeMute ?? 1.0;
+        }
+    }
+
     private void ToggleMicMute()
     {
         if (MicMuted)