fix(local-library): plex id double-prefix + scrim z-index over buttons

Two bugs from the same session:

  - The plex_aggregated CTE prefixed `group_key` with `'plex:'`, but
    `plex/mod.rs::plex_album_key()` already returns `"plex:<hash>"`.
    Result: album.id was `"plex:plex:<hash>"` from the chunked path,
    which `handleAlbumClick` couldn't resolve, so opening an album
    showed NO TRACKS. Fixed: use `COALESCE(album_key, 'plex:' ||
    rating_key)` so populated rows pass through untouched and only
    the rating_key fallback gets the prefix added. GROUP BY mirrors
    the same expression for consistency.

  - On AlbumCardLibraryLite the scrim (`.cover-wrap::after`) and the
    overlay container (`.overlay`) were both at `z-index: 2`. Pseudo-
    elements paint after all DOM children at the same z-index, so
    the scrim ended up ON TOP of the buttons, hiding them on hover.
    Re-laid the layering so it's monotonic top-to-bottom:
        1  cover img
        2  scrim
        3  overlay (action buttons)
        4  select-checkbox + source-badge-slot

Plus: restored the dark `rgba(0,0,0,0.85)` hover scrim itself,
removed the `::after` accent-border ring + title-colour cue. Per the
2026-05-12 user call ("Hasta quisiera el overlay de regreso") — the
Plex thumbnail downscale (9f417999) made the scrim's opacity
transition cheap again since the underlying paint is now 220x220
not 1000x1000. Buttons go back to opacity 0 → 1 on hover with a
single transition on `.action-buttons`. White-outline circles
(transparent fill) since the scrim provides contrast.

cc

crates/qbz-library/src/database.rs

@@ -2005,7 +2005,10 @@ impl LibraryDatabase {
             r#",
             plex_aggregated AS (
                 SELECT
-                    'plex:' || COALESCE(album_key, rating_key) AS group_key,
+                    -- `album_key` is populated by plex/mod.rs::plex_album_key()
+                    -- which already returns `"plex:<hash>"`. Only the
+                    -- rating_key fallback needs the prefix added.
+                    COALESCE(album_key, 'plex:' || rating_key) AS group_key,
                     COALESCE(album, 'Unknown Album') AS title,
                     CASE WHEN COUNT(DISTINCT artist) > 1
                          THEN 'Various Artists'
@@ -2023,7 +2026,7 @@ impl LibraryDatabase {
                     CAST(NULL AS TEXT) AS source_folders,
                     'plex' AS source
                 FROM plex_cache.plex_cache_tracks
-                GROUP BY COALESCE(album_key, rating_key)
+                GROUP BY COALESCE(album_key, 'plex:' || rating_key)
             )"#
         } else {
             ""
@@ -2205,11 +2208,14 @@ impl LibraryDatabase {
             r#",
             plex_aggregated AS (
                 SELECT
-                    'plex:' || COALESCE(album_key, rating_key) AS group_key,
+                    -- `album_key` is populated by plex/mod.rs::plex_album_key()
+                    -- which already returns `"plex:<hash>"`. Only the
+                    -- rating_key fallback needs the prefix added.
+                    COALESCE(album_key, 'plex:' || rating_key) AS group_key,
                     COALESCE(album, 'Unknown Album') AS title,
                     COALESCE(MIN(artist), 'Unknown Artist') AS artist
                 FROM plex_cache.plex_cache_tracks
-                GROUP BY COALESCE(album_key, rating_key)
+                GROUP BY COALESCE(album_key, 'plex:' || rating_key)
             )"#
         } else {
             ""