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/discover/discover_screen.dart

@@ -1,3 +1,4 @@
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:dio/dio.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -204,9 +205,14 @@ class _ResultTile extends StatelessWidget {
             color: fs.slate,
             child: row.imageUrl.isEmpty
                 ? Icon(Icons.album, color: fs.ash)
-                : Image.network(row.imageUrl, fit: BoxFit.cover,
-                    errorBuilder: (_, __, ___) =>
-                        Icon(Icons.album, color: fs.ash)),
+                : CachedNetworkImage(
+                    imageUrl: row.imageUrl,
+                    fit: BoxFit.cover,
+                    fadeInDuration: Duration.zero,
+                    fadeOutDuration: Duration.zero,
+                    errorWidget: (_, __, ___) =>
+                        Icon(Icons.album, color: fs.ash),
+                  ),
           ),
         ),
         const SizedBox(width: 12),