Billing upgrade/downgrade, inviti in app e diretta YouTube da mobile.

Upgrade Stripe con addebito immediato; downgrade programmato a fine periodo.
UX billing più sobria con box info; icone piani. API inviti e OAuth YouTube
per staff trasmissione; deep link join; scelta partita programmata o nuova.

Co-authored-by: Cursor <cursoragent@cursor.com>

mobile/lib/core/invite_storage.dart

@@ -0,0 +1,20 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+class InviteStorage {
+  static const _keyToken = 'pending_invite_token';
+
+  static Future<void> saveToken(String token) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_keyToken, token);
+  }
+
+  static Future<String?> readToken() async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getString(_keyToken);
+  }
+
+  static Future<void> clear() async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.remove(_keyToken);
+  }
+}