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/lib/player/player_bar.dart

@@ -1,6 +1,7 @@
 import 'dart:io';
 
 import 'package:audio_service/audio_service.dart';
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
@@ -96,10 +97,15 @@ class _TrackInfo extends StatelessWidget {
     // so the transition works regardless of what's playing.
     Widget cover;
     if (media.artUri != null) {
+      // CachedNetworkImageProvider for HTTPS art URIs so the mini bar
+      // hits the same disk cache the rest of the UI uses (ServerImage,
+      // discover thumbnails). FileImage stays for the file:// branch
+      // populated by AlbumCoverCache — bytes are already on disk and a
+      // second cache layer would only burn duplicate space.
       cover = Image(
         image: media.artUri!.isScheme('file')
             ? FileImage(File.fromUri(media.artUri!)) as ImageProvider
-            : NetworkImage(media.artUri.toString()),
+            : CachedNetworkImageProvider(media.artUri.toString()),
         width: 48,
         height: 48,
         // Without a fit, Image paints the source at its intrinsic