전체 UI 색상 통일 - 제각각이던 빨강/주황/파랑 계열을 의미별 토큰 4종으로 정리

danger/warning/success/approved(+각 Bg 버전)를 AppPalette에 추가하고,
화면마다 다르게 쓰이던 Colors.red[400/600/700]/redAccent, orange/amber
계열, blue 계열 리터럴을 전부 해당 토큰으로 교체. NFC 체크인 화면의
홀로 튀던 Colors.blueAccent도 함께 정리.

lib/ui/teacher_student_management_page.dart

@@ -223,7 +223,10 @@ class _TeacherStudentManagementPageState
                   child: SingleChildScrollView(
                     child: Text(
                       result.failures.join('\n'),
-                      style: const TextStyle(fontSize: 12, color: Colors.red),
+                      style: const TextStyle(
+                        fontSize: 12,
+                        color: AppPalette.danger,
+                      ),
                     ),
                   ),
                 ),
@@ -321,7 +324,7 @@ class _TeacherStudentManagementPageState
         shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(24)),
         title: Row(
           children: [
-            Icon(Icons.warning_amber_rounded, color: Colors.red[700]),
+            Icon(Icons.warning_amber_rounded, color: AppPalette.danger),
             const SizedBox(width: 10),
             const Text(
               '계정 완전 삭제',
@@ -339,7 +342,7 @@ class _TeacherStudentManagementPageState
           ),
           ElevatedButton(
             style: ElevatedButton.styleFrom(
-              backgroundColor: Colors.red[600],
+              backgroundColor: AppPalette.danger,
               foregroundColor: Colors.white,
             ),
             onPressed: () async {
@@ -519,10 +522,10 @@ class _TeacherStudentManagementPageState
           height: double.infinity,
           padding: const EdgeInsets.all(20),
           decoration: BoxDecoration(
-            color: _isDragging ? Colors.orange[50] : AppPalette.paper,
+            color: _isDragging ? AppPalette.warningBg : AppPalette.paper,
             borderRadius: BorderRadius.circular(24),
             border: Border.all(
-              color: _isDragging ? Colors.orange : AppPalette.sage,
+              color: _isDragging ? AppPalette.warning : AppPalette.sage,
               width: _isDragging ? 2 : 1,
             ),
           ),
@@ -532,7 +535,7 @@ class _TeacherStudentManagementPageState
               Icon(
                 Icons.upload_file_rounded,
                 size: 40,
-                color: _isDragging ? Colors.orange : AppPalette.sage,
+                color: _isDragging ? AppPalette.warning : AppPalette.sage,
               ),
               const SizedBox(height: 12),
               Text(
@@ -540,7 +543,7 @@ class _TeacherStudentManagementPageState
                 textAlign: TextAlign.center,
                 style: TextStyle(
                   fontWeight: FontWeight.bold,
-                  color: _isDragging ? Colors.orange[800] : AppPalette.ink,
+                  color: _isDragging ? AppPalette.warning : AppPalette.ink,
                 ),
               ),
               const SizedBox(height: 4),
@@ -584,15 +587,13 @@ class _TeacherStudentManagementPageState
           Container(
             padding: const EdgeInsets.all(8),
             decoration: BoxDecoration(
-              color: needsReset
-                  ? Colors.red.withValues(alpha: 0.1)
-                  : AppPalette.linen,
+              color: needsReset ? AppPalette.dangerBg : AppPalette.linen,
               shape: BoxShape.circle,
             ),
             child: Icon(
               needsReset ? Icons.lock_reset : Icons.person,
               size: 18,
-              color: needsReset ? Colors.red : AppPalette.ink,
+              color: needsReset ? AppPalette.danger : AppPalette.ink,
             ),
           ),
           const SizedBox(height: 8),
@@ -609,7 +610,7 @@ class _TeacherStudentManagementPageState
             textAlign: TextAlign.center,
             style: TextStyle(
               fontSize: 11.5,
-              color: needsReset ? Colors.red[700] : Colors.grey[600],
+              color: needsReset ? AppPalette.danger : Colors.grey[600],
               fontWeight: needsReset ? FontWeight.bold : FontWeight.normal,
             ),
           ),
@@ -650,7 +651,7 @@ class _TeacherStudentManagementPageState
               IconButton(
                 icon: const Icon(
                   Icons.delete_forever_rounded,
-                  color: Colors.redAccent,
+                  color: AppPalette.danger,
                   size: 18,
                 ),
                 tooltip: '계정 영구 삭제',