M3u playlist export

[timusus]

This commit implements a robust, scalable, and accurate solution for exporting playlists to m3u format as requested in issue #112.

Core Components:

• M3uWriter: Generates m3u file content from song lists with proper formatting

  • Supports extended m3u format with #EXTINF metadata
  • Handles duration conversion (milliseconds to seconds)
  • Provides custom path resolver support for flexibility

• PlaylistExporter: Service class for handling export operations

  • Multiple export strategies: to URI, to directory, or generate content
  • Best-effort path resolution for content:// URIs
  • Robust error handling with detailed error messages
  • Uses Android Storage Access Framework for file operations

Repository Layer:

• Added export methods to PlaylistRepository interface
• Implemented in LocalPlaylistRepository with proper song retrieval

UI Layer:

• Added "Export playlist" menu item to playlist detail screen
• Integrated with Android's file picker using ActivityResultContracts
• User-friendly success/error messages with localized strings
• Export uses playlist name as default filename

Technical Approach:

• Handles Android's content:// URI limitation by attempting to resolve real paths
• Falls back to content URIs when real paths unavailable (known limitation)
• Flexible path resolver allows callers to provide custom resolution logic
• Comprehensive error handling for permissions, I/O, and invalid data

Testing:

• Added comprehensive unit tests for M3uWriter covering:
  • Basic single/multiple song export
  • Null handling for artist/track names
  • Custom path resolvers
  • Duration conversion accuracy
  • M3u format structure validation

Addresses issue #112