fix(mpris): enrich metadata across equivalent players

[arqueon]

Summary

• keep the active MPRIS source stable and prefer a controllable player over informational mirrors
• group equivalent playing sources and use the richest peer for stable title, artist, and album metadata
• let TrackArtService borrow artwork from an equivalent MPRIS source
• show the stable album value in the native media player

Why

Browsers and desktop integrations can expose multiple MPRIS players for the same playback. One source may provide working transport controls but incomplete metadata, while another exposes the artist, album, or artwork. Selecting only the first playing entry leaves the native DMS media UI with missing information.

The fix is provider-agnostic: it operates only on MPRIS state and capabilities and adds no site-specific lookup or scraping.

Impact

DMS remains the owner of player selection and transport. The native media surfaces and any plugin consuming MprisController or TrackArtService can use richer metadata without switching control to a read-only mirror. Single-source players keep their existing behavior.

Validation

• Qt 6 qmllint passed for all three changed QML files
• live reload completed without new QML errors
• verified with simultaneous controllable, metadata-rich, and read-only MPRIS sources: artist and album remained correct and TrackArtService.artReadyFor() returned true
• confirmed the patch adds no provider, domain, or HTML-scraping logic