feat: add persistent cover image cache

- Add CoverCacheManager service for persistent image caching
- Cache stored in app_flutter/cover_cache/ (not cleared by system)
- Maximum 1000 images cached for up to 365 days
- Update all 11 screens to use persistent cache manager
- Add flutter_cache_manager and path dependencies
- Update CHANGELOG.md with all changes for v3.1.3

lib/widgets/cached_cover_image.dart

@@ -0,0 +1,69 @@
+import 'package:flutter/material.dart';
+import 'package:cached_network_image/cached_network_image.dart';
+import 'package:spotiflac_android/services/cover_cache_manager.dart';
+
+/// A wrapper around CachedNetworkImage that uses persistent cache storage.
+/// 
+/// This ensures cover images are cached to disk and persist across app restarts,
+/// instead of being stored in the temporary directory that can be cleared by the OS.
+class CachedCoverImage extends StatelessWidget {
+  final String imageUrl;
+  final double? width;
+  final double? height;
+  final BoxFit fit;
+  final int? memCacheWidth;
+  final int? memCacheHeight;
+  final Widget Function(BuildContext, String, Object)? errorWidget;
+  final Widget Function(BuildContext, String)? placeholder;
+  final BorderRadius? borderRadius;
+
+  const CachedCoverImage({
+    super.key,
+    required this.imageUrl,
+    this.width,
+    this.height,
+    this.fit = BoxFit.cover,
+    this.memCacheWidth,
+    this.memCacheHeight,
+    this.errorWidget,
+    this.placeholder,
+    this.borderRadius,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final image = CachedNetworkImage(
+      imageUrl: imageUrl,
+      width: width,
+      height: height,
+      fit: fit,
+      memCacheWidth: memCacheWidth,
+      memCacheHeight: memCacheHeight,
+      cacheManager: CoverCacheManager.isInitialized 
+          ? CoverCacheManager.instance 
+          : null,
+      errorWidget: errorWidget,
+      placeholder: placeholder,
+    );
+
+    if (borderRadius != null) {
+      return ClipRRect(
+        borderRadius: borderRadius!,
+        child: image,
+      );
+    }
+
+    return image;
+  }
+}
+
+/// Provider for CachedNetworkImageProvider that uses persistent cache.
+/// Use this for precacheImage() calls.
+CachedNetworkImageProvider cachedCoverImageProvider(String url) {
+  return CachedNetworkImageProvider(
+    url,
+    cacheManager: CoverCacheManager.isInitialized 
+        ? CoverCacheManager.instance 
+        : null,
+  );
+}