Library-source polish: reconciliation UX, target parity, export edge cases #382

Follow-up umbrella after the Plex/Jellyfin/Emby library-source rollout. User-selectable music library sections already landed for Plex (#377) and Jellyfin/Emby (#379); this tracks the remaining polish:

• Reconciliation review UX improvements (bulk actions, clearer diff states)
• Target parity: ensure every library source exposes the same sync/health surface
• Playlist/export edge cases across sources (empty libraries, renamed sections, permission-limited accounts)

Sub-issues can be split out as scope firms up.