feat: i18n 개선 및 userfront 로그인/로케일 보완

userfront/lib/core/i18n/locale_gate.dart

@@ -0,0 +1,51 @@
+import 'package:easy_localization/easy_localization.dart' hide tr;
+import 'package:flutter/material.dart';
+
+import 'package:userfront/i18n.dart';
+import '../services/web_window.dart';
+import 'locale_storage.dart';
+import 'locale_utils.dart';
+
+class LocaleGate extends StatefulWidget {
+  const LocaleGate({super.key, required this.localeCode, required this.child});
+
+  final String localeCode;
+  final Widget child;
+
+  @override
+  State<LocaleGate> createState() => _LocaleGateState();
+}
+
+class _LocaleGateState extends State<LocaleGate> {
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    _applyLocale();
+  }
+
+  @override
+  void didUpdateWidget(LocaleGate oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (oldWidget.localeCode != widget.localeCode) {
+      _applyLocale();
+    }
+  }
+
+  Future<void> _applyLocale() async {
+    final normalized = normalizeLocaleCode(widget.localeCode);
+    LocaleStorage.write(normalized);
+    webWindow.setTitle(
+      tr('ui.userfront.app_title'),
+    );
+    if (context.locale.languageCode == normalized) {
+      return;
+    }
+    await context.setLocale(Locale(normalized));
+    webWindow.setTitle(
+      tr('ui.userfront.app_title'),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) => widget.child;
+}