feat: aggiungi controllo aggiornamenti con dialog all'avvio

All'avvio l'app chiama l'API GitHub releases per confrontare la versione
installata con l'ultima disponibile (comparazione semantica major.minor.patch).
Se esiste una versione più recente, mostra un AlertDialog con i pulsanti
"Più tardi" e "Scarica" — quest'ultimo apre releases/latest nel browser.
Il controllo è silenzioso in caso di assenza di rete o errori, e disabilitato
su Flutter web (dart:io non disponibile).

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

lib/shared/services/update_service.dart

@@ -0,0 +1,55 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter/foundation.dart' show kIsWeb, visibleForTesting;
+
+import '../../core/constants/app_constants.dart';
+
+class UpdateService {
+  static const _apiUrl =
+      'https://api.github.com/repos/davide3011/biteplan/releases/latest';
+
+  /// Returns the latest version string if it's newer than the current version,
+  /// or null if already up to date or if the check fails.
+  static Future<String?> checkUpdate() async {
+    if (kIsWeb) return null;
+    try {
+      final client = HttpClient()
+        ..connectionTimeout = const Duration(seconds: 5);
+      final request = await client
+          .getUrl(Uri.parse(_apiUrl))
+          .timeout(const Duration(seconds: 5));
+      request.headers
+        ..set('Accept', 'application/vnd.github+json')
+        ..set('User-Agent', 'biteplan/$kAppVersion');
+      final response =
+          await request.close().timeout(const Duration(seconds: 5));
+      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'), '');
+      return isNewer(tag, kAppVersion) ? tag : null;
+    } catch (_) {
+      return null;
+    }
+  }
+
+  @visibleForTesting
+  static bool isNewer(String remote, String local) {
+    final r = parseVersion(remote);
+    final l = parseVersion(local);
+    for (var i = 0; i < 3; i++) {
+      if (r[i] > l[i]) return true;
+      if (r[i] < l[i]) return false;
+    }
+    return false;
+  }
+
+  @visibleForTesting
+  static List<int> parseVersion(String v) {
+    final parts = v.split('.');
+    return List.generate(
+        3, (i) => i < parts.length ? (int.tryParse(parts[i]) ?? 0) : 0);
+  }
+}

lib/shared/services/url_launcher_service.dart

@@ -0,0 +1,9 @@
+import 'package:flutter/services.dart';
+
+class UrlLauncherService {
+  static const _channel = MethodChannel('com.davide.biteplan/launcher');
+
+  static Future<void> launch(String url) async {
+    await _channel.invokeMethod<void>('launch', {'url': url});
+  }
+}