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 {

app/src/test/kotlin/com/subtracks/ui/theme/ArtworkExtractionTest.kt

@@ -4,6 +4,10 @@ import android.content.Context
 import android.graphics.Bitmap
 import android.graphics.Color
 import androidx.activity.ComponentActivity
+import androidx.compose.material3.Text
+import androidx.compose.runtime.MutableState
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
 import androidx.compose.ui.graphics.luminance
 import androidx.compose.ui.test.junit4.v2.createAndroidComposeRule
 import androidx.test.core.app.ApplicationProvider
@@ -15,6 +19,7 @@ import coil3.asImage
 import coil3.test.FakeImageLoaderEngine
 import com.subtracks.data.model.CoverArtRef
 import org.junit.After
+import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertTrue
 import org.junit.Before
@@ -64,4 +69,22 @@ class ArtworkExtractionTest {
         assertNotNull(colors)
         assertTrue(colors!!.scheme.background.luminance() < 0.1f)
     }
+
+    @Test
+    fun keepsContentStateWhenArtworkArrives() {
+        val artwork = mutableStateOf<ArtworkColors?>(null)
+        lateinit var state: MutableState<Int>
+        composeRule.setContent {
+            SubtracksTheme {
+                ArtworkTheme(artwork.value) {
+                    state = remember { mutableStateOf(0) }
+                    Text(state.value.toString())
+                }
+            }
+        }
+        composeRule.runOnIdle { state.value = 42 }
+        composeRule.runOnIdle { artwork.value = artworkColorsFromSeed(0xFF3A7BD5.toInt()) }
+        composeRule.waitForIdle()
+        assertEquals(42, state.value)
+    }
 }