fix(locallibrary): show offline/folder album covers + add source badge

Two fixes from the first Albums-grid test pass.

Offline/folder covers (the reported "cover exists in the folder but isn't
shown"): some albums — offline-cached CMAF bundles and folder downloads —
have no artwork_path in the index even though a cover.jpg sits in the track
folder (older offline-cache downloads didn't backfill it). Add a frontend-
agnostic LibraryDatabase::resolve_album_cover_fallback(group_key) that finds
a representative track, derives its folder (the path itself for a CMAF bundle
dir, else the parent), and returns <folder>/cover.jpg when it exists. The
Slint Albums mapper calls it whenever the album has no artwork. Verified
against the live index: it resolves the folder covers that exist on disk
(FLAC-layout + CMAF-with-cover). Older CMAF bundles that never downloaded a
cover (segments only) still show the placeholder — those need a cover
re-fetch (offline-cache maintenance), tracked separately.

Source badge: the Local Library album card now shows the always-visible
source badge inside the artwork (bottom-right) — a hard-drive glyph for local
files, the Qobuz logo for offline copies, the Plex logo for Plex. Added
AlbumCard.show-source-badge (default off; threaded through AlbumCollectionView
/AlbumGrid) + AlbumCard.source; album_map now carries the real source and
map_local_album maps user->local / qobuz_download / plex. Small rounded
square, not a pill (ADR-008). Logos copied into the Slint icon assets.

Built green (cargo check -p qbz-slint); runtime smoke-test pending.

cc

crates/qbz-slint/ui/locallibrary/LocalLibraryView.slint

@@ -364,6 +364,8 @@ export component LocalLibraryView inherits Rectangle {
                                 show-source: false;
                                 // Local albums aren't Qobuz-favoritable — hide the heart.
                                 show-favorite: false;
+                                // Always-visible source badge (local | qobuz-offline | plex).
+                                show-source-badge: true;
                                 open-album(id) => {
                                     LocalLibraryActions.open-album(id);
                                 }