feat: adopt material 3 theme and billing adjustments

lib/widgets/common/form_fields/base_text_field.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
-import '../../../theme/app_colors.dart';
+// import '../../../theme/app_colors.dart';
 
 /// 공통 텍스트 필드 위젯
 /// 프로젝트 전체에서 일관된 스타일의 텍스트 입력 필드를 제공합니다.
@@ -66,10 +66,10 @@ class BaseTextField extends StatelessWidget {
         if (label != null) ...[
           Text(
             label!,
-            style: const TextStyle(
+            style: TextStyle(
               fontSize: 16,
               fontWeight: FontWeight.w600,
-              color: AppColors.textSecondary,
+              color: Theme.of(context).colorScheme.onSurfaceVariant,
             ),
           ),
           const SizedBox(height: 8),
@@ -89,22 +89,22 @@ class BaseTextField extends StatelessWidget {
           maxLines: maxLines,
           minLines: minLines,
           readOnly: readOnly,
-          cursorColor: cursorColor ?? theme.primaryColor,
+          cursorColor: cursorColor ?? theme.colorScheme.primary,
           style: style ??
-              const TextStyle(
+              TextStyle(
                 fontSize: 16,
-                color: AppColors.textPrimary,
+                color: Theme.of(context).colorScheme.onSurface,
               ),
           decoration: InputDecoration(
             hintText: hintText,
-            hintStyle: const TextStyle(
-              color: AppColors.textMuted,
+            hintStyle: TextStyle(
+              color: Theme.of(context).colorScheme.onSurfaceVariant,
             ),
             prefixIcon: prefixIcon,
             prefixText: prefixText,
             suffixIcon: suffixIcon,
             filled: true,
-            fillColor: fillColor ?? AppColors.surfaceColorAlt,
+            fillColor: fillColor ?? Theme.of(context).colorScheme.surface,
             contentPadding: contentPadding ?? const EdgeInsets.all(16),
             border: OutlineInputBorder(
               borderRadius: BorderRadius.circular(16),
@@ -113,15 +113,15 @@ class BaseTextField extends StatelessWidget {
             focusedBorder: OutlineInputBorder(
               borderRadius: BorderRadius.circular(16),
               borderSide: BorderSide(
-                color: theme.primaryColor,
+                color: theme.colorScheme.primary,
                 width: 2,
               ),
             ),
             enabledBorder: OutlineInputBorder(
               borderRadius: BorderRadius.circular(16),
               borderSide: BorderSide(
-                color: AppColors.borderColor.withValues(alpha: 0.7),
-                width: 1.5,
+                color: theme.colorScheme.outline.withValues(alpha: 0.6),
+                width: 1,
               ),
             ),
             disabledBorder: OutlineInputBorder(