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_call_screen.dart

@@ -376,7 +376,7 @@ class _TeacherCallScreenState extends State<TeacherCallScreen> {
         final location = t['location'] ?? '교무실';
         final isSelected = _selectedTeacherId == t['teacherId'];
         final inClass = _controller.isInClassNow(name);
-        final statusColor = inClass ? Colors.orange : AppPalette.ink;
+        final statusColor = inClass ? AppPalette.warning : AppPalette.ink;
 
         return GestureDetector(
           onTap: () => setState(() {
@@ -428,17 +428,21 @@ class _TeacherCallScreenState extends State<TeacherCallScreen> {
                         decoration: BoxDecoration(
                           color: isSelected
                               ? Colors.white.withValues(alpha: 0.15)
-                              : Colors.orange[50],
+                              : AppPalette.warningBg,
                           borderRadius: BorderRadius.circular(20),
                           border: Border.all(
-                            color: isSelected ? Colors.white54 : Colors.orange,
+                            color: isSelected
+                                ? Colors.white54
+                                : AppPalette.warning,
                           ),
                         ),
                         child: Text(
                           '수업중',
                           style: TextStyle(
                             fontSize: 9,
-                            color: isSelected ? Colors.white : Colors.orange,
+                            color: isSelected
+                                ? Colors.white
+                                : AppPalette.warning,
                           ),
                         ),
                       ),