keep the theme wrapper stable so the layout doesn't remount when artwork arrives

ArtworkTheme swapped between a bare content() and a MaterialTheme wrapper depending on
whether a scheme existed. When the first track's artwork (or a screen's) resolved, that
branch change remounted the whole subtree, reloading images, resetting scroll and
restarting the mini player. Always provide a MaterialTheme, falling back to the ambient
scheme when there is no artwork, and cover it with a test.

app/src/main/kotlin/com/subtracks/ui/theme/ArtworkTheme.kt

@@ -204,12 +204,11 @@ fun ArtworkTheme(
     colors: ArtworkColors?,
     content: @Composable () -> Unit,
 ) {
-    val scheme = colors?.scheme
-    if (scheme == null) {
-        content()
-    } else {
-        MaterialTheme(colorScheme = scheme, typography = SubtracksTypography, content = content)
-    }
+    MaterialTheme(
+        colorScheme = colors?.scheme ?: MaterialTheme.colorScheme,
+        typography = SubtracksTypography,
+        content = content,
+    )
 }
 
 fun playerSurfaceColor(artwork: ArtworkColors): Color {