feat(flutter): disk-persistent cover cache via cached_network_image

Slice 1 of the cover-caching pass. The previous Image.network /
NetworkImage path only cached covers in memory, so a scroll-off + scroll-
back or an app restart re-downloaded every tile from the server. Swap
to cached_network_image so bytes land on disk (path_provider temp dir,
URL-keyed) and survive both.

Sites migrated:
  - ServerImage (all /api/*/cover usage — home grid, library, playlist,
    artist/album detail headers)
  - DiscoverScreen Lidarr suggestion thumbnails
  - PlayerBar mini cover (HTTPS branch; file:// branch unchanged since
    AlbumCoverCache files are already on disk)

Auth header forwarding preserved via httpHeaders. Fade-in disabled so
populated grids paint instantly on cache hit.

Slice 2 (pre-warm during sync) builds on this same cache manager.

flutter_client/pubspec.yaml

@@ -30,6 +30,13 @@ dependencies:
   connectivity_plus: ^6.0.5
   flutter_timezone: ^4.1.1
   palette_generator: ^0.3.3
+  # Disk-persistent image cache. Image.network only caches in memory, so
+  # cover art repainted after a scroll-off or app restart re-fetches from
+  # the server. cached_network_image stores the bytes under
+  # path_provider's temp dir keyed by URL, surviving both. Used directly
+  # by ServerImage (auth-aware path) and as CachedNetworkImageProvider
+  # for the mini bar (which composes its own Image widget).
+  cached_network_image: ^3.4.1
 
 dev_dependencies:
   flutter_test: