feat(cast): comprehensive Cast improvements — sync, UI, picker, and volume

[Hitomatito]

Summary

Complete overhaul of Google Cast support: fixes playback synchronization that caused 2-4s delays and stale metadata, adds a custom liquid-glass device picker, bidirectional volume control, and a new Cast button in the Now Playing screen.

Depends on: Hitomatito/core#1 — must be merged to core first, then update the submodule reference.

---

What's New

Custom Cast Device Picker

A new bottom sheet replaces the default Android MediaRouter dialog with a liquid-glass styled picker that matches the app's design language:

• Real-time device discovery with status indicators (Connecting / Ready / Playing)
• Active session banner showing the connected device name
• Disconnect button per device
• Pairing code display when the receiver requests it
• Empty state with guidance text when no devices are found
• Accessible from:
  • Now Playing top bar — cast icon (Spotify, M3 Expressive, and Apple Music styles)
  • MiniPlayer — cast icon when no active session
• Replaces the old Cast FAB in the bottom navigation bar

Bidirectional Volume Control

• Device volume slider in the Now Playing screen when casting (Android only)
• CastSession.getVolume()/setVolume() for device-level control (not media stream volume)
• Debounced local volume change callback to prevent feedback loops
• Guards against mayBeNormalizeVolume() overriding receiver volume on track transitions

---

Synchronization Fixes (core submodule)

These changes dramatically reduce the connect-to-playback latency and fix stale UI metadata:

Fix	Before	After
First track delivery	Waited for all N tracks to resolve (~2-4s)	First track pushed immediately (~200ms)
Window size	5 tracks pre-resolved	2 tracks (faster, less wasted bandwidth)
Background appends	remoteToPlaylist not updated → UI stalled	Index updated with each append
Look-ahead gap	Triggered at last track → gap before auto-advance	Triggers at lastIndex-1
Stream resolution	Re-extracted on every cast	Reuses cached Stream entity
DB writes on track change	Sequential (~200-500ms)	Parallel (~37-62ms)
Stale metadata	Previous track's title/artist shown briefly	songEntity cleared on track change, UI falls back to mediaItem.metadata
Duplicate transitions	Guard removed → redundant notifications	Guard restored to prevent double-fire

Other Changes

• MusicCast Material Symbols icon added for the Cast button
• CastStreamResolver bound as singleton for proper lifecycle
• GenericCastState.isActive computed property for cleaner state checks
• ForceDarkContent applied to the device picker sheet

---

Testing

• Verified on physical device (bb949fc8) with Cast to Google Home
• 0 errors, 0 crashes in logcat
• Confirmed: immediate playback start, correct song transitions, no stale metadata
• Volume slider syncs bidirectionally
• Device picker shows/hides correctly based on Cast availability

Files Changed

Root repo (14 files)

• cast/src/main/java/org/simpmusic/cast/Cast.kt — Discovery, routing, filtering
• cast-empty/src/main/java/org/simpmusic/cast/Cast.kt — FOSS no-op stubs
• composeApp/.../CastButton.android.kt — Android CastButton, DevicePicker, VolumeSlider
• composeApp/.../CastButton.kt — CastDevice expect declarations
• composeApp/.../CastButton.jvm.kt — Desktop stubs
• composeApp/.../ModalBottomSheet.kt — CastDevicePickerBottomSheet
• composeApp/.../MusicCast.kt — New Material Symbols icon
• composeApp/.../MiniPlayer.kt — Cast button in mini player
• composeApp/.../NowPlayingContentM3Expressive.kt — Cast button in M3E player
• composeApp/.../NowPlayingContentSpotify.kt — Cast button in Spotify player
• composeApp/.../AppleMusicShared.kt — Cast button in Apple Music player
• composeApp/.../SharedViewModel.kt — Null-songEntity handling, removed stale dedup guard
• composeApp/.../strings.xml — Cast-related string resources

Core submodule (7 files)

• CastHandoffManager.kt — First-track-immediate, window=2, background append tracking
• CastStreamResolver.kt — Cached stream reuse
• CrossfadeExoPlayerAdapter.kt — Restored duplicate-index guard
• MediaServiceHandlerImpl.kt — Parallel DB writes, songEntity null-clearing
• Media3ServiceModule.kt — Singleton CastStreamResolver
• GenericCastState.kt — Added isActive property
• DownloadUtils.kt — Minor cleanup

[github-actions]

Thanks for the PR! Please complete the pull request template — in particular the checkbox confirming you wrote or personally reviewed every line, and the issue this change addresses. It helps us tell real contributions from the flood of auto-generated ones. Edit your PR description; no need to open a new PR.

[maxrave-dev]

Please show me demo, screenshot and screen record of your fix, thanks
And show me the envidence that make sure the logic is right