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).

Services/LayoutStore.Save.cs

@@ -54,8 +54,10 @@ public partial class LayoutStore : IDisposable
         using (var cmd = _connection.CreateCommand())
         {
             cmd.CommandText = """
-                INSERT INTO Scene (Id, Name, IsHidden, IsChatScene, HasBackground, HasSocialBar, ThumbnailAssetId, SortOrder)
-                VALUES ($id, $name, $isHidden, $isChat, $hasBackground, $hasSocialBar, $thumbnail, $sort)
+                INSERT INTO Scene (Id, Name, IsHidden, IsChatScene, HasBackground, HasSocialBar, ThumbnailAssetId,
+                                   SceneMutesMic, SceneMutesDesktop, SortOrder)
+                VALUES ($id, $name, $isHidden, $isChat, $hasBackground, $hasSocialBar, $thumbnail,
+                        $mutesMic, $mutesDesktop, $sort)
                 """;
             cmd.Transaction = tx;
             var idP = cmd.Parameters.Add("$id", SqliteType.Text);
@@ -65,6 +67,8 @@ public partial class LayoutStore : IDisposable
             var backgroundP = cmd.Parameters.Add("$hasBackground", SqliteType.Integer);
             var socialBarP = cmd.Parameters.Add("$hasSocialBar", SqliteType.Integer);
             var thumbnailP = cmd.Parameters.Add("$thumbnail", SqliteType.Text);
+            var mutesMicP = cmd.Parameters.Add("$mutesMic", SqliteType.Integer);
+            var mutesDesktopP = cmd.Parameters.Add("$mutesDesktop", SqliteType.Integer);
             var sortP = cmd.Parameters.Add("$sort", SqliteType.Integer);
 
             var sort = 0;
@@ -77,6 +81,12 @@ public partial class LayoutStore : IDisposable
                 backgroundP.Value = scene.HasBackground ? 1 : 0;
                 socialBarP.Value = scene.HasSocialBar ? 1 : 0;
                 thumbnailP.Value = (object?)scene.ThumbnailAssetId ?? DBNull.Value;
+                mutesMicP.Value = scene.SceneMutesMic.HasValue
+                    ? (object)(scene.SceneMutesMic.Value ? 1 : 0)
+                    : DBNull.Value;
+                mutesDesktopP.Value = scene.SceneMutesDesktop.HasValue
+                    ? (object)(scene.SceneMutesDesktop.Value ? 1 : 0)
+                    : DBNull.Value;
                 sortP.Value = sort++;
                 cmd.ExecuteNonQuery();
             }