Add Gitea update notifications

lib/backend/models/app_update_info.dart

@@ -0,0 +1,23 @@
+class AppUpdateInfo {
+  const AppUpdateInfo({
+    required this.version,
+    required this.tagName,
+    required this.name,
+    required this.body,
+    required this.htmlUrl,
+    required this.publishedAt,
+    this.apkUrl,
+    this.windowsInstallerUrl,
+  });
+
+  final String version;
+  final String tagName;
+  final String name;
+  final String body;
+  final String htmlUrl;
+  final DateTime? publishedAt;
+  final String? apkUrl;
+  final String? windowsInstallerUrl;
+
+  bool get hasAssets => apkUrl != null || windowsInstallerUrl != null;
+}