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_detail_controller.dart

@@ -13,11 +13,17 @@ class GroupDetailController extends ChangeNotifier {
   bool _isLoading = true;
   bool _isBusy = false;
   List<dynamic> _members = [];
+  List<dynamic> _joinRequests = [];
+  bool _isPublic = false;
   bool _disposed = false;
 
   bool get isLoading => _isLoading;
   bool get isBusy => _isBusy;
   List<dynamic> get members => _members;
+  List<dynamic> get joinRequests => _joinRequests;
+  bool get isPublic => _isPublic;
+  bool get isOwner =>
+      _members.any((m) => m['studentId'] == studentId && m['role'] == 'owner');
 
   void _safeNotify() {
     if (!_disposed) notifyListeners();
@@ -41,11 +47,77 @@ class GroupDetailController extends ChangeNotifier {
       if (response.statusCode == 200) {
         final data = jsonDecode(utf8.decode(response.bodyBytes));
         _members = data['members'] ?? [];
+        _isPublic = data['isPublic'] == true;
       }
     } catch (_) {
       // 조용히 무시
+    }
+    if (isOwner) await _fetchJoinRequests();
+    _isLoading = false;
+    _safeNotify();
+  }
+
+  Future<void> _fetchJoinRequests() async {
+    try {
+      final response = await http.get(
+        Uri.parse(
+          '$baseUrl/api/groups/$groupId/join-requests?ownerId=$studentId',
+        ),
+      );
+      if (response.statusCode == 200) {
+        final data = jsonDecode(utf8.decode(response.bodyBytes));
+        _joinRequests = data['requests'] ?? [];
+      }
+    } catch (_) {
+      // 조용히 무시
+    }
+  }
+
+  Future<(bool success, String message)> respondJoinRequest(
+    int requestId,
+    bool accept,
+  ) async {
+    _isBusy = true;
+    _safeNotify();
+    try {
+      final response = await http.post(
+        Uri.parse('$baseUrl/api/groups/join-requests/$requestId/respond'),
+        headers: {"Content-Type": "application/json"},
+        body: jsonEncode({"responderId": studentId, "accept": accept}),
+      );
+      final result = jsonDecode(utf8.decode(response.bodyBytes));
+      if (response.statusCode == 200 && result['status'] == 'success') {
+        await refresh();
+        return (true, '${result['message'] ?? '처리했습니다.'}');
+      }
+      return (false, '${result['message'] ?? '처리 실패'}');
+    } catch (e) {
+      return (false, '네트워크 에러: $e');
     } finally {
-      _isLoading = false;
+      _isBusy = false;
+      _safeNotify();
+    }
+  }
+
+  Future<(bool success, String message)> setVisibility(bool isPublic) async {
+    _isBusy = true;
+    _safeNotify();
+    try {
+      final response = await http.post(
+        Uri.parse('$baseUrl/api/groups/$groupId/visibility'),
+        headers: {"Content-Type": "application/json"},
+        body: jsonEncode({"ownerId": studentId, "isPublic": isPublic}),
+      );
+      final result = jsonDecode(utf8.decode(response.bodyBytes));
+      if (response.statusCode == 200 && result['status'] == 'success') {
+        await refresh();
+        return (true, '${result['message'] ?? '변경했습니다.'}');
+      }
+      return (false, '${result['message'] ?? '변경 실패'}');
+    } catch (e) {
+      return (false, '네트워크 에러: $e');
+    } finally {
+      _isBusy = false;
       _safeNotify();
     }
   }