simplified code across the app and consolidated duplicated helpers

lib/widget/avatar_disk_cache.dart

@@ -66,13 +66,15 @@ class AvatarDiskCache {
     return isGroup ? 'g_$hex' : 'u_${hex}_$size';
   }
 
-  static String _hex(String value) {
-    final buffer = StringBuffer();
-    for (final b in utf8.encode(value)) {
-      buffer.write(b.toRadixString(16).padLeft(2, '0'));
-    }
-    return buffer.toString();
-  }
+  static String _hex(String value) =>
+      utf8.encode(value).map((b) => b.toRadixString(16).padLeft(2, '0')).join();
+
+  File _file(
+    String dirPath, {
+    required String id,
+    required bool isGroup,
+    required int size,
+  }) => File('$dirPath/${fileName(id: id, isGroup: isGroup, size: size)}');
 
   /// Synchronous read for warm sessions (cache directory already resolved).
   /// Returns null when the directory isn't known yet — the caller falls back
@@ -86,9 +88,7 @@ class AvatarDiskCache {
     final path = _dirPath;
     if (path == null) return null;
     try {
-      final file = File(
-        '$path/${fileName(id: id, isGroup: isGroup, size: size)}',
-      );
+      final file = _file(path, id: id, isGroup: isGroup, size: size);
       if (!file.existsSync()) return null;
       final bytes = file.readAsBytesSync();
       return bytes.isEmpty ? null : bytes;
@@ -104,9 +104,7 @@ class AvatarDiskCache {
   }) async {
     try {
       final dir = await _directory();
-      final file = File(
-        '${dir.path}/${fileName(id: id, isGroup: isGroup, size: size)}',
-      );
+      final file = _file(dir.path, id: id, isGroup: isGroup, size: size);
       if (!file.existsSync()) return null;
       final bytes = await file.readAsBytes();
       return bytes.isEmpty ? null : bytes;
@@ -123,10 +121,12 @@ class AvatarDiskCache {
   }) async {
     try {
       final dir = await _directory();
-      final file = File(
-        '${dir.path}/${fileName(id: id, isGroup: isGroup, size: size)}',
-      );
-      await file.writeAsBytes(bytes, flush: true);
+      await _file(
+        dir.path,
+        id: id,
+        isGroup: isGroup,
+        size: size,
+      ).writeAsBytes(bytes);
       if (!_pruned) {
         _pruned = true;
         unawaited(_prune(dir));
@@ -142,7 +142,7 @@ class AvatarDiskCache {
     try {
       final dir = await _directory();
       if (isGroup) {
-        final file = File('${dir.path}/${fileName(id: id, isGroup: true, size: 0)}');
+        final file = _file(dir.path, id: id, isGroup: true, size: 0);
         if (file.existsSync()) await file.delete();
         return;
       }