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.lock

@@ -121,6 +121,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "8.12.6"
+  cached_network_image:
+    dependency: "direct main"
+    description:
+      name: cached_network_image
+      sha256: "7c1183e361e5c8b0a0f21a28401eecdbde252441106a9816400dd4c2b2424916"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.4.1"
+  cached_network_image_platform_interface:
+    dependency: transitive
+    description:
+      name: cached_network_image_platform_interface
+      sha256: "35814b016e37fbdc91f7ae18c8caf49ba5c88501813f73ce8a07027a395e2829"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.1"
+  cached_network_image_web:
+    dependency: transitive
+    description:
+      name: cached_network_image_web
+      sha256: "980842f4e8e2535b8dbd3d5ca0b1f0ba66bf61d14cc3a17a9b4788a3685ba062"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.3.1"
   characters:
     dependency: transitive
     description:
@@ -672,6 +696,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "9.3.0"
+  octo_image:
+    dependency: transitive
+    description:
+      name: octo_image
+      sha256: "34faa6639a78c7e3cbe79be6f9f96535867e879748ade7d17c9b1ae7536293bd"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.1.0"
   package_config:
     dependency: transitive
     description: