refactor: estrai showUpdateDialog e parseTagName in file dedicati

Sposta la logica del dialog in lib/shared/widgets/update_dialog.dart
per renderla importabile nei test senza duplicare il codice.
Estrae parseTagName come metodo @visibleForTesting in UpdateService
isolando il parsing JSON dalla chiamata HTTP. app.dart semplificato
di conseguenza.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/shared/services/update_service.dart

@@ -27,14 +27,24 @@ class UpdateService {
       final body = await response.transform(utf8.decoder).join();
       client.close();
       if (response.statusCode != 200) return null;
-      final data = jsonDecode(body) as Map<String, dynamic>;
-      final tag = (data['tag_name'] as String).replaceFirst(RegExp(r'^v'), '');
+      final tag = parseTagName(body);
+      if (tag == null) return null;
       return isNewer(tag, kAppVersion) ? tag : null;
     } catch (_) {
       return null;
     }
   }
 
+  @visibleForTesting
+  static String? parseTagName(String body) {
+    try {
+      final data = jsonDecode(body) as Map<String, dynamic>;
+      return (data['tag_name'] as String).replaceFirst(RegExp(r'^v'), '');
+    } catch (_) {
+      return null;
+    }
+  }
+
   @visibleForTesting
   static bool isNewer(String remote, String local) {
     final r = parseVersion(remote);