학교 전용 공부 타이머(열품타) + 학년별 랭킹 기능 추가

학생 참여 유도를 위해 공부 시작/종료 타이머와 학년별(오늘/이번주/전체)
랭킹 화면을 대시보드에 추가. 로그인 응답에 학년 정보를 포함시켜
세션 저장/복원 전체 경로에 전달되도록 함.

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

lib/ui/login_screen.dart

@@ -78,6 +78,7 @@ class _LoginScreenState extends State<LoginScreen> {
           result.name!,
           result.role!,
           result.isDeviceMatched,
+          result.grade,
         );
         break;
       case LoginOutcome.success:
@@ -87,6 +88,7 @@ class _LoginScreenState extends State<LoginScreen> {
           result.studentId!,
           result.name!,
           result.isDeviceMatched,
+          result.grade,
         );
         break;
     }
@@ -98,6 +100,7 @@ class _LoginScreenState extends State<LoginScreen> {
     String name,
     String role,
     bool isDeviceMatched,
+    int? grade,
   ) {
     final TextEditingController newPwController = TextEditingController();
 
@@ -168,6 +171,7 @@ class _LoginScreenState extends State<LoginScreen> {
                               studentId,
                               name,
                               isDeviceMatched,
+                              grade,
                             );
                           } else {
                             AppNotice.show(context, message);
@@ -189,6 +193,7 @@ class _LoginScreenState extends State<LoginScreen> {
     String studentId,
     String name,
     bool isDeviceMatched,
+    int? grade,
   ) async {
     if (_keepLoggedIn) {
       await SessionStore.save(
@@ -197,6 +202,7 @@ class _LoginScreenState extends State<LoginScreen> {
           userName: name,
           role: role,
           isDeviceMatched: isDeviceMatched,
+          grade: grade,
         ),
       );
     } else {
@@ -211,6 +217,7 @@ class _LoginScreenState extends State<LoginScreen> {
           userName: name,
           role: role,
           isDeviceMatched: isDeviceMatched,
+          grade: grade,
         ),
       ),
     );