Add offline song download feature

[timusus]

…by, Plex)

This implements comprehensive offline download functionality for remote media providers, allowing users to download songs for offline playback similar to Spotify.

Core Features

Database Schema

• Added DownloadState enum to track download states (NONE, QUEUED, DOWNLOADING, PAUSED, COMPLETED, FAILED)
• Created DownloadData entity with Room database support
• Added DownloadDao with comprehensive query methods
• Incremented database version to 41
• Added TypeConverter for DownloadState

Download Repository

• Created DownloadRepository interface for download operations
• Implemented DownloadRepositoryImpl with full CRUD operations
• Supports queuing, progress tracking, pause/resume, and cancellation
• Includes automatic file cleanup when downloads are removed

Download Manager

• Implemented DownloadManager singleton for coordinating downloads
• Supports concurrent downloads (max 3 simultaneous)
• Automatic queue processing
• Progress tracking and error handling
• File storage in app-specific directories organized by provider

Provider Integration

• Jellyfin: Added buildJellyfinDownloadPath() for direct download URLs (no transcoding)
• Emby: Added buildEmbyDownloadPath() for direct download URLs
• Plex: Added buildPlexDownloadPath() for direct download URLs
• All download URLs use static=true to avoid transcoding and get original files

Offline Playback

• Updated JellyfinMediaInfoProvider to check for offline files first
• Updated EmbyMediaInfoProvider to check for offline files first
• Updated PlexMediaInfoProvider to check for offline files first
• Seamless fallback to streaming if file not downloaded
• Returns file:// URIs for downloaded content (isRemote = false)

Dependency Injection

• Added DownloadRepository binding in RepositoryModule
• Injected DownloadRepository into all MediaInfoProviders
• DownloadManager uses constructor injection with Hilt

Architecture

The implementation follows the existing codebase patterns:

• Repository pattern for data access
• Hilt/Dagger for dependency injection
• Room for database operations
• Coroutines for async operations
• Clean separation between data, domain, and presentation layers

Storage Structure

/data/data/com.simplecityapps.shuttle/files/downloads/
├── jellyfin/
│   └── {itemId}.{ext}
├── emby/
│   └── {itemId}.{ext}
└── plex/
    └── {itemId}.{ext}

Next Steps (for future PRs)

1. Implement DownloadService for background downloads with notifications
2. Add UI components (download buttons, progress indicators)
3. Implement batch download for albums/playlists
4. Add download settings (WiFi-only, storage location, quality)
5. Implement storage management and cleanup UI
6. Add download notifications with progress
7. Handle network changes and retry logic
8. Add tests for download functionality

Addresses #88