그룹 스터디 공개/비공개 기능 추가

그룹을 공개로 만들면 "공개 그룹" 목록에 뜨고, 학생이 참여 신청을
보내면 그룹장이 허용/거절할 수 있다. 그룹 상세 화면에 그룹장 전용
공개/비공개 전환 버튼과 대기 중인 참여 신청 목록을 추가.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/function/group_controller.dart

@@ -14,12 +14,14 @@ class GroupController extends ChangeNotifier {
   bool _isBusy = false;
   List<dynamic> _groups = [];
   List<dynamic> _invites = [];
+  List<dynamic> _publicGroups = [];
   bool _disposed = false;
 
   bool get isLoading => _isLoading;
   bool get isBusy => _isBusy;
   List<dynamic> get groups => _groups;
   List<dynamic> get invites => _invites;
+  List<dynamic> get publicGroups => _publicGroups;
 
   void _safeNotify() {
     if (!_disposed) notifyListeners();
@@ -36,7 +38,7 @@ class GroupController extends ChangeNotifier {
   Future<void> refresh() async {
     _isLoading = true;
     _safeNotify();
-    await Future.wait([_fetchGroups(), _fetchInvites()]);
+    await Future.wait([_fetchGroups(), _fetchInvites(), _fetchPublicGroups()]);
     _isLoading = false;
     _safeNotify();
   }
@@ -69,7 +71,24 @@ class GroupController extends ChangeNotifier {
     }
   }
 
-  Future<(bool success, String message)> createGroup(String name) async {
+  Future<void> _fetchPublicGroups() async {
+    try {
+      final response = await http.get(
+        Uri.parse('$baseUrl/api/groups/public?studentId=$studentId'),
+      );
+      if (response.statusCode == 200) {
+        final data = jsonDecode(utf8.decode(response.bodyBytes));
+        _publicGroups = data['groups'] ?? [];
+      }
+    } catch (_) {
+      // 조용히 무시
+    }
+  }
+
+  Future<(bool success, String message)> createGroup(
+    String name, {
+    bool isPublic = false,
+  }) async {
     _isBusy = true;
     _safeNotify();
     try {
@@ -80,6 +99,7 @@ class GroupController extends ChangeNotifier {
           "ownerId": studentId,
           "ownerName": studentName,
           "name": name.trim(),
+          "isPublic": isPublic,
         }),
       );
       final result = jsonDecode(utf8.decode(response.bodyBytes));
@@ -121,4 +141,27 @@ class GroupController extends ChangeNotifier {
       _safeNotify();
     }
   }
+
+  Future<(bool success, String message)> requestJoin(int groupId) async {
+    _isBusy = true;
+    _safeNotify();
+    try {
+      final response = await http.post(
+        Uri.parse('$baseUrl/api/groups/$groupId/join-request'),
+        headers: {"Content-Type": "application/json"},
+        body: jsonEncode({"studentId": studentId, "studentName": studentName}),
+      );
+      final result = jsonDecode(utf8.decode(response.bodyBytes));
+      if (response.statusCode == 200 && result['status'] == 'success') {
+        await _fetchPublicGroups();
+        return (true, '${result['message'] ?? '참여 신청을 보냈습니다.'}');
+      }
+      return (false, '${result['message'] ?? '참여 신청 실패'}');
+    } catch (e) {
+      return (false, '네트워크 에러: $e');
+    } finally {
+      _isBusy = false;
+      _safeNotify();
+    }
+  }
 }