feat: add authorized YouTube audio downloads

[RetricSu]

Summary

• add a yt-dlp based service for downloading authorized YouTube audio as local MP3 files with embedded metadata and thumbnails
• add a player-side download entry in the playback info area with URL input and destination folder selection
• auto-add or resync the download folder after completion so downloaded tracks appear in the library

Verification

• cargo fmt
• cargo check
• cargo test
• cargo clippy -- -D warnings

Notes

• Requires yt-dlp and ffmpeg to be installed and available in PATH at runtime.
• The UI copy frames this as authorized audio downloads only.

[gemini-code-assist]

Code Review

This pull request introduces a feature to download authorized audio from YouTube using yt-dlp. It adds a new YoutubeDownloadService to handle the download process asynchronously, integrates it with the application state and event loop, and provides a user-friendly download dialog in the playback info panel with support for localization. The code review feedback highlights several important improvements: enhancing cross-platform compatibility on Windows by checking the USERPROFILE environment variable, securing the yt-dlp command execution against option injection using --, simplifying the library path lookup logic to avoid fragile assumptions, and localizing hardcoded English status messages to align with the application's internationalization standards.

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