Add Spotify huddle controls

[klopez4212]

Summary

• Adds Spotify OAuth connection management under Settings > Connections.
• Adds Spotify Premium playback controls to huddles, including DJ ownership/handoff and shared playback state events.
• Adds the playing-state music-note animation around the huddle Spotify control.

Screenshot

Checks

• cd desktop && ../bin/pnpm typecheck
• cd desktop && ../bin/pnpm check
• cd desktop && ../bin/pnpm test
• bin/cargo check --manifest-path desktop/src-tauri/Cargo.toml
• bin/cargo fmt --all --check
• pre-commit hooks
• pre-push hooks