fix: cover art on auto-advance + native back-button overlay routing

Issue 1 (MusicService): wrap DefaultHttpDataSource in CacheBitmapLoader so
Media3 notification artwork is fetched once and served from memory on every
subsequent auto-advance. warmArtworkCache() pre-loads current+next 3 tracks
on both onMediaItemTransition and onTimelineChanged (queue replace via setQueue).
Adds explicit media3-datasource dependency.

Issue 2 (back button): replace deprecated onKeyDown with OnBackPressedCallback.
WebBridge.setOverlayState(bool) tracks whether JS has an overlay open;
consumeBackPress() is called first — if true, MainActivity delegates to
window.NSPlayer.handleBack() which closes the topmost layer. Falls back to
webView.goBack() then moveTaskToBack(true) so the MusicService binding survives.
JS side: _syncOverlayState() + MutationObserver keep the native flag current;
handleBack covers NP overlay, queue panel, action sheet, all modals, sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -44,5 +44,6 @@ dependencies {
     implementation(libs.media3.exoplayer)
     implementation(libs.media3.session)
     implementation(libs.media3.common)
+    implementation(libs.media3.datasource)
     implementation(libs.kotlinx.coroutines.android)
 }