Let page background shine through spotlight tiles

By giving spotlight tiles a transparent background in certain layouts.

src/state/TileViewModel.ts

@@ -37,6 +37,7 @@ export class SpotlightTileViewModel {
   public constructor(
     public readonly media$: Behavior<MediaViewModel[]>,
     public readonly maximised$: Behavior<boolean>,
+    public readonly bgStyle$: Behavior<"solid" | "transparent">,
   ) {}
 }