Use Premium YouTube audio automatically and show a Premium badge

[dariusnv]

Summary

• automatically request the signed-in account's highest-bitrate audio-only stream, including Premium AAC and Opus, so subscribers use the quality included in their subscription without another setting
• fall back to ordinary audio if authenticated resolution or downloading fails; keep guest playback unchanged
• enable WebM audio decoding for YouTube Opus streams and record the selected codec and reported bitrate at debug level; download no video
• show a small Premium badge beside YouTube Music in Settings → General after the session confirms High-tier audio; clear confirmation on account/session changes and make no extra membership requests
• use ytmusic-rs #5, currently pinned to the tested fork commit; update the dependency to its upstream revision once that companion PR merges
• verify 279 tests, formatting and clean Clippy on the current upstream Sonora base, plus 36 tests in the companion library; live Windows checks cover Premium AAC/Opus download, decoding and seeking, invalid-authentication fallback to ordinary AAC, and the badge's placement in the built app
• keep this separate from Add Discord Rich Presence, YouTube account photos, and audio math fixes #443; note that the badge confirms Premium audio availability after a track is resolved, rather than querying billing status, and macOS/Linux runtime behavior was not manually tested