1. Discover button on undiscovered playlist cards — triggers discovery
directly from Explorer instead of redirecting to Sync page. Button
changes to "Open" to reopen modal after closing.
2. Status badges on playlist cards: checkmark (in library), heart
(wishlisted), star (fully discovered), percentage (needs discovery).
Meta line shows "N in library · M wishlisted" counts.
3. Auto-refresh: polls every 5s during active discovery to update cards.
WebSocket listener for discovery:progress events. Cards refresh when
discovery completes.
4. Explored tracking: playlists get green checkmark badge after tree is
built (session-only, resets on reload).
Backend: new get_mirrored_playlist_status_counts with fail-safe design —
core discovery counts use simple reliable queries, library/wishlist
counts are best-effort extras that won't break discovery detection.
Card layout redesigned: badges inline with playlist name, discover
button below meta text, no more absolute positioning overlaps.
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