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/gradle/libs.versions.toml

@@ -14,6 +14,7 @@ material = { group = "com.google.android.material", name = "material", version.r
 media3-exoplayer = { group = "androidx.media3", name = "media3-exoplayer", version.ref = "media3" }
 media3-session = { group = "androidx.media3", name = "media3-session", version.ref = "media3" }
 media3-common = { group = "androidx.media3", name = "media3-common", version.ref = "media3" }
+media3-datasource = { group = "androidx.media3", name = "media3-datasource", version.ref = "media3" }
 kotlinx-coroutines-android = { group = "org.jetbrains.kotlinx", name = "kotlinx-coroutines-android", version.ref = "coroutines" }
 
 [plugins]