전역 테마를 팔레트로 교체 + 빠졌던 팝업 화면들 마무리

앱 전체 MaterialApp의 ThemeData가 여전히 primarySwatch: Colors.indigo
였던 게 진짜 원인이었음 — 개별 화면 색은 고쳤지만, 기본 위젯 스타일
(포커스 안 된 텍스트필드 밑줄, FilterChip 선택 색 등)은 전부 이
전역 테마를 그대로 따라가고 있어서 여전히 예전 색으로 보였음.
ColorScheme을 AppPalette 기준으로 재구성해 해결.

추가로 팔레트 작업에서 빠졌던 화면들도 마무리:
- teacher_register_screen.dart (교사 회원가입)
- nfc_tag_writer_screen.dart (NFC 태그 쓰기)
- change_password_screen.dart (미사용이지만 일관성을 위해 정리)
- teacher_attendance_page.dart의 주머니 번호 칩 색상

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

lib/ui/nfc_tag_writer_screen.dart

@@ -2,6 +2,7 @@
 // 버튼/입력폼을 그린다. NFC 세션/서버 통신은 lib/function/nfc_tag_writer_controller.dart가 담당한다.
 import 'package:flutter/material.dart';
 import '../function/nfc_tag_writer_controller.dart';
+import '../theme/app_palette.dart';
 
 class NfcTagWriterScreen extends StatefulWidget {
   const NfcTagWriterScreen({super.key});
@@ -49,10 +50,11 @@ class _NfcTagWriterScreenState extends State<NfcTagWriterScreen> {
       builder: (context, _) {
         final bool isWriting = _controller.isWriting;
         return Scaffold(
+          backgroundColor: AppPalette.mist,
           appBar: AppBar(
             title: const Text("NFC 주머니 태그 쓰기"),
-            backgroundColor: Colors.deepPurple,
-            foregroundColor: Colors.white,
+            backgroundColor: AppPalette.ink,
+            foregroundColor: AppPalette.paper,
           ),
           body: Padding(
             padding: const EdgeInsets.all(24.0),
@@ -73,7 +75,7 @@ class _NfcTagWriterScreenState extends State<NfcTagWriterScreen> {
                 Icon(
                   isWriting ? Icons.nfc : Icons.edit_note_rounded,
                   size: 80,
-                  color: isWriting ? Colors.orange : Colors.deepPurple,
+                  color: isWriting ? Colors.orange : AppPalette.ink,
                 ),
                 const SizedBox(height: 16),
                 Text(
@@ -87,8 +89,8 @@ class _NfcTagWriterScreenState extends State<NfcTagWriterScreen> {
                   height: 55,
                   child: ElevatedButton(
                     style: ElevatedButton.styleFrom(
-                      backgroundColor: Colors.deepPurple,
-                      foregroundColor: Colors.white,
+                      backgroundColor: AppPalette.ink,
+                      foregroundColor: AppPalette.paper,
                     ),
                     onPressed: isWriting
                         ? null

lib/ui/teacher_attendance_page.dart

@@ -442,15 +442,15 @@ class _TeacherAttendancePageState extends State<TeacherAttendancePage> {
                     vertical: 3,
                   ),
                   decoration: BoxDecoration(
-                    color: Colors.blue.shade50,
+                    color: AppPalette.linen,
                     borderRadius: BorderRadius.circular(20),
-                    border: Border.all(color: Colors.blue.shade200),
+                    border: Border.all(color: AppPalette.sage),
                   ),
                   child: Text(
                     student['pocketNumber'],
                     style: const TextStyle(
                       fontWeight: FontWeight.bold,
-                      color: Colors.blueAccent,
+                      color: AppPalette.ink,
                       fontSize: 11,
                     ),
                   ),

lib/ui/teacher_register_screen.dart

@@ -2,6 +2,7 @@
 // lib/function/teacher_register_controller.dart가 담당한다.
 import 'package:flutter/material.dart';
 import '../function/teacher_register_controller.dart';
+import '../theme/app_palette.dart';
 
 class TeacherRegisterScreen extends StatefulWidget {
   const TeacherRegisterScreen({super.key});
@@ -61,10 +62,11 @@ class _TeacherRegisterScreenState extends State<TeacherRegisterScreen> {
       listenable: _controller,
       builder: (context, _) {
         return Scaffold(
+          backgroundColor: AppPalette.mist,
           appBar: AppBar(
             title: const Text('교사 회원가입'),
-            backgroundColor: Colors.indigo,
-            foregroundColor: Colors.white,
+            backgroundColor: AppPalette.ink,
+            foregroundColor: AppPalette.paper,
           ),
           body: SingleChildScrollView(
             padding: const EdgeInsets.all(24.0),
@@ -122,8 +124,8 @@ class _TeacherRegisterScreenState extends State<TeacherRegisterScreen> {
                   height: 50,
                   child: ElevatedButton(
                     style: ElevatedButton.styleFrom(
-                      backgroundColor: Colors.indigo,
-                      foregroundColor: Colors.white,
+                      backgroundColor: AppPalette.ink,
+                      foregroundColor: AppPalette.paper,
                     ),
                     onPressed: _controller.isLoading ? null : _registerTeacher,
                     child: _controller.isLoading