백품타 랭킹에 전체랭킹/학년랭킹 필터 추가

grade 쿼리파라미터를 생략하면 전체 학년 통합 랭킹을 주도록 백엔드를
바꾸고, 랭킹 화면에 "N학년"/"전체랭킹" 필 필터를 추가해 시간대
필터(오늘/이번주/전체)와 별도로 고를 수 있게 했다.

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

lib/function/study_ranking_controller.dart

@@ -11,10 +11,12 @@ class StudyRankingController extends ChangeNotifier {
 
   bool _isLoading = true;
   String _period = 'today'; // today / week / total
+  String _scope = 'grade'; // grade / all
   List<dynamic> _ranking = [];
 
   bool get isLoading => _isLoading;
   String get period => _period;
+  String get scope => _scope;
   List<dynamic> get ranking => _ranking;
 
   Future<void> init() => fetchRanking();
@@ -25,12 +27,19 @@ class StudyRankingController extends ChangeNotifier {
     await fetchRanking();
   }
 
+  Future<void> setScope(String scope) async {
+    if (_scope == scope) return;
+    _scope = scope;
+    await fetchRanking();
+  }
+
   Future<void> fetchRanking() async {
     _isLoading = true;
     notifyListeners();
     try {
+      final gradeParam = _scope == 'grade' ? '&grade=$grade' : '';
       final response = await http.get(
-        Uri.parse('$baseUrl/api/study/ranking?grade=$grade&period=$_period'),
+        Uri.parse('$baseUrl/api/study/ranking?period=$_period$gradeParam'),
       );
       if (response.statusCode == 200) {
         final data = jsonDecode(utf8.decode(response.bodyBytes));