Feat(backup): implement advanced app state backup (v2) & dynamic locale hot-reload

[YisusPineapple]

Description

This pull request elevates Lune's backup utility from a simple playlist exporter to a robust, full-featured Application State Backup & Restore Engine (Backup v2). It ensures complete data preservation (including favorites, custom covers, and listening history), resolves UI configuration/language lifecycle bugs, and delivers native Material 3 Expressive visual feedback.

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Key Enhancements

1. Advanced Application State Backup (Backup v2 Schema)

• Song Overrides & Favorites: Integrated full serialization of the SQLite song_overrides database entities [MusicDatabase.kt]. Restoring a backup now seamlessly recovers custom metadata (titles, artists, genres), custom local covers, and all liked/favorite statuses (isFavorite = true).
• App Settings: Serializes the entire state of SettingsManager using a strongly-typed, nested data structure (SettingsBackupData). This avoids generic GSON parsing issues (such as numbers defaulting to doubles) and guarantees exact restoration of audio, theme, and UI preferences.
• Accumulated Statistics: Exports and imports the SQLite playback_stats table to ensure that home screen recommendation widgets and listening statistics are preserved across devices.
• Backward Compatibility: Fully guaranteed. If a legacy v1 backup file is uploaded, the parser gracefully defaults to playlist-only restoration without throwing exceptions.

2. Runtime Localization & Theming Hot-Reload

• Programmatically writing settings to SharedPreferences during import did not force Android's localization engine to update the active UI layout.
• The Fix: On import completion, the engine on the Main Thread dynamically invokes AppCompatDelegate.setApplicationLocales(LocaleListCompat.forLanguageTags(code)) with the newly restored language code, followed by an immediate activity.recreate() call. This dynamically re-loads the entire layout tree (applying theme, custom colors, and language) without requiring a manual app restart.

3. Material 3 Expressive Loader & Thread Stabilization

• Replaced instantaneous, blank-screen recreation toasts with a rich, non-dismissible Material 3 AlertDialog during file I/O operations.
• The loader is driven by a lightweight, hardware-accelerated CircularWavyProgressIndicator powered by a smooth infinite loop transition, making it extremely friendly on modest devices.
• Introduced a two-step loading phase: "Importing..." ➔ "Finalizing restoration..." backed by a 1200ms coroutine delay. This guarantees SQLite write-ahead logging (WAL) and journal files on disk are fully flushed and synchronized before the Activity context is torn down and recreated.

4. Spanish Localization Alignment

• Corrected inconsistent English labels on Spanish layouts (values-es/strings.xml):
  • "Cover player" ➔ "Carátula del reproductor"
  • "Control player" ➔ "Controles del reproductor"
  • "Export/Import Playlists" ➔ "Exportar/Importar copia de seguridad"
• Fully localized standard playback actions (Shuffle, Timer, Lyrics, Automix, etc.) for a clean, premium Spanish-language experience.

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Technical Details & Architecture

• Minimum SDK: 24 (Retrocompatible up to Android 15 / API 36).
• Concurrency: Heavy I/O disk writes and Room database operations are strictly bound to Kotlin Coroutines running on Dispatchers.IO to protect Dispatchers.Main from frame drops or UI janks.
• R8 Minification Safety: All newly introduced backup-specific data structures are fully shielded from obfuscation via existing ProGuard rules (-keep class com.demonlab.lune.tools.** { *; }).

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Testing Performed

• Database Clear Test: Performed database wipe, restored the advanced v2 JSON, and verified that playlists, custom colors, equalizer presets, home screen statistics, and song favorite stars restored successfully.
• Legacy Test: Verified that legacy v1 backup files (containing only playlists) are still parsed and restored successfully.
• Language Reload Test: Verified that importing a backup containing German/Spanish settings dynamically hot-reloads the UI in the target language.