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);

lib/shared/widgets/update_dialog.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+import '../services/url_launcher_service.dart';
+
+const _releasesUrl = 'https://github.com/davide3011/biteplan/releases/latest';
+
+void showUpdateDialog(BuildContext context, String newVersion) {
+  showDialog<void>(
+    context: context,
+    builder: (ctx) => AlertDialog(
+      icon: const Icon(Icons.system_update_outlined, size: 32),
+      title: const Text('Aggiornamento disponibile'),
+      content: Text(
+        'È disponibile la versione $newVersion di BitePlan.\n'
+        'Vuoi scaricare il nuovo APK?',
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.of(ctx).pop(),
+          child: const Text('Più tardi'),
+        ),
+        FilledButton(
+          onPressed: () {
+            Navigator.of(ctx).pop();
+            UrlLauncherService.launch(_releasesUrl);
+          },
+          child: const Text('Scarica'),
+        ),
+      ],
+    ),
+  );
+}