학생/교사/관리자 대시보드를 하나의 공통 화면으로 통합

StudentDashboard, TeacherDashboard, AdminDashboard(진입 라우팅)를
MainDashboard 하나로 합치고, 역할(role)에 따라 배너 색상/문구와
보이는 타일만 달라지게 함. 그리드 레이아웃/카드 디자인은 이제
한 곳에서만 관리되어, 앞으로 UI를 고치면 모든 역할에 자동 적용됨.

- 2061(개발자) 계정에 기존에 빠져있던 "스마트기기 반출 대장" 타일 추가
- role=='admin' 실 계정도 이제 학생 계정 관리/출석 확인 등 교사 기능에
  접근 가능 (관리자는 교사 권한을 포함하는 것으로 정리)
- 작은 DB 초기화 전용 화면(admin_dashboard.dart)은 "서버 DB 제어"
  타일의 목적지로만 유지, 로그인 직후 랜딩 화면 역할은 제거

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

lib/ui/login_screen.dart

@@ -3,9 +3,7 @@
 import 'package:flutter/material.dart';
 import '../config.dart' show schoolName;
 import '../function/login_controller.dart';
-import 'student_dashboard.dart';
-import 'teacher_dashboard.dart';
-import 'admin_dashboard.dart';
+import 'main_dashboard.dart';
 import 'teacher_register_screen.dart';
 
 // -------------------------------------------------------------
@@ -50,9 +48,10 @@ class _LoginScreenState extends State<LoginScreen> {
         Navigator.pushReplacement(
           context,
           MaterialPageRoute(
-            builder: (context) => StudentDashboard(
-              studentId: result.studentId!,
-              studentName: result.name!,
+            builder: (context) => MainDashboard(
+              userId: result.studentId!,
+              userName: result.name!,
+              role: 'student',
               isDeviceMatched: true,
             ),
           ),
@@ -154,9 +153,9 @@ class _LoginScreenState extends State<LoginScreen> {
 
                           if (success) {
                             Navigator.pop(dialogContext);
-                            ScaffoldMessenger.of(context).showSnackBar(
-                              SnackBar(content: Text(message)),
-                            );
+                            ScaffoldMessenger.of(
+                              context,
+                            ).showSnackBar(SnackBar(content: Text(message)));
                             _navigateBasedOnRole(
                               role,
                               studentId,
@@ -164,9 +163,9 @@ class _LoginScreenState extends State<LoginScreen> {
                               isDeviceMatched,
                             );
                           } else {
-                            ScaffoldMessenger.of(context).showSnackBar(
-                              SnackBar(content: Text(message)),
-                            );
+                            ScaffoldMessenger.of(
+                              context,
+                            ).showSnackBar(SnackBar(content: Text(message)));
                           }
                         },
                         child: const Text('변경하고 시작하기'),
@@ -186,31 +185,17 @@ class _LoginScreenState extends State<LoginScreen> {
     String name,
     bool isDeviceMatched,
   ) {
-    if (role == 'teacher') {
-      Navigator.pushReplacement(
-        context,
-        MaterialPageRoute(
-          builder: (context) =>
-              TeacherDashboard(teacherId: studentId, teacherName: name),
+    Navigator.pushReplacement(
+      context,
+      MaterialPageRoute(
+        builder: (context) => MainDashboard(
+          userId: studentId,
+          userName: name,
+          role: role,
+          isDeviceMatched: isDeviceMatched,
         ),
-      );
-    } else if (role == 'admin') {
-      Navigator.pushReplacement(
-        context,
-        MaterialPageRoute(builder: (context) => const AdminDashboard()),
-      );
-    } else {
-      Navigator.pushReplacement(
-        context,
-        MaterialPageRoute(
-          builder: (context) => StudentDashboard(
-            studentId: studentId,
-            studentName: name,
-            isDeviceMatched: isDeviceMatched,
-          ),
-        ),
-      );
-    }
+      ),
+    );
   }
 
   void _showErrorDialog(String message) {
@@ -403,7 +388,7 @@ class _LoginScreenState extends State<LoginScreen> {
                           onPressed: () => _showLegacyPasswordDialog(
                             '선생님',
                             '1234',
-                            const TeacherDashboard(),
+                            const MainDashboard(role: 'teacher'),
                           ),
                         ),
                         TextButton.icon(
@@ -419,7 +404,7 @@ class _LoginScreenState extends State<LoginScreen> {
                           onPressed: () => _showLegacyPasswordDialog(
                             '시스템 관리자',
                             '4936',
-                            const AdminDashboard(),
+                            const MainDashboard(role: 'admin'),
                           ),
                         ),
                       ],