fix(auth): list missing Twitch scopes in the re-login dialog

Existing tokens lack the newer mod scopes, so Helix 401s looked like a vague permissions failure. Cache scopes from /oauth2/validate and show which ones need re-authorization.

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

test/utils/twitch_oauth_scopes_test.dart

@@ -0,0 +1,66 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frosty/utils/twitch_oauth_scopes.dart';
+
+void main() {
+  group('missingTwitchScopes', () {
+    test('returns scopes not present on the token', () {
+      final missing = missingTwitchScopes(
+        granted: const ['chat:read', 'chat:edit'],
+      );
+
+      expect(missing, contains('user:read:moderated_channels'));
+      expect(missing, contains('moderator:manage:chat_messages'));
+      expect(missing, contains('moderator:manage:banned_users'));
+      expect(missing, isNot(contains('chat:read')));
+    });
+
+    test('returns empty when all required scopes are granted', () {
+      expect(
+        missingTwitchScopes(granted: twitchUserScopes),
+        isEmpty,
+      );
+    });
+  });
+
+  group('unauthorizedDialogMessage', () {
+    test('explains expired session when logged out', () {
+      expect(
+        unauthorizedDialogMessage(
+          isLoggedIn: false,
+          grantedScopes: const [],
+        ),
+        contains('expired'),
+      );
+    });
+
+    test('lists human-readable missing permissions when logged in', () {
+      final message = unauthorizedDialogMessage(
+        isLoggedIn: true,
+        grantedScopes: const [
+          'chat:read',
+          'chat:edit',
+          'user:read:follows',
+          'user:read:blocked_users',
+          'user:manage:blocked_users',
+          'user:manage:chat_color',
+        ],
+      );
+
+      expect(message, contains('additional Twitch permissions'));
+      expect(message, contains('View channels you moderate'));
+      expect(message, contains('Delete chat messages as a moderator'));
+      expect(message, contains('Timeout and ban users as a moderator'));
+      expect(message, isNot(contains('Send chat messages')));
+    });
+
+    test('falls back when logged in but no missing scopes are known', () {
+      expect(
+        unauthorizedDialogMessage(
+          isLoggedIn: true,
+          grantedScopes: twitchUserScopes,
+        ),
+        contains('missing permissions'),
+      );
+    });
+  });
+}