feat: adopt material 3 theme and billing adjustments

lib/widgets/common/buttons/secondary_button.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import '../../../theme/app_colors.dart';
 
 /// 부차적인 액션에 사용되는 Secondary 버튼
 /// 취소, 되돌아가기, 부가 옵션 등에 사용됩니다.
@@ -42,15 +41,17 @@ class _SecondaryButtonState extends State<SecondaryButton> {
 
   @override
   Widget build(BuildContext context) {
-    final effectiveBorderColor = widget.borderColor ?? AppColors.secondaryColor;
-    final effectiveTextColor = widget.textColor ?? AppColors.primaryColor;
+    final theme = Theme.of(context);
+    final effectiveBorderColor =
+        widget.borderColor ?? theme.colorScheme.outline;
+    final effectiveTextColor = widget.textColor ?? theme.colorScheme.primary;
 
     Widget button = AnimatedContainer(
       duration: const Duration(milliseconds: 200),
       width: widget.width,
       height: widget.height,
       transform: widget.enableHoverEffect && _isHovered
-          ? (Matrix4.identity()..scale(1.02))
+          ? Matrix4.diagonal3Values(1.02, 1.02, 1.0)
           : Matrix4.identity(),
       child: OutlinedButton(
         onPressed: widget.onPressed,
@@ -70,8 +71,9 @@ class _SecondaryButtonState extends State<SecondaryButton> {
                 vertical: 12,
                 horizontal: 24,
               ),
-          backgroundColor:
-              _isHovered ? AppColors.glassBackground : Colors.transparent,
+          backgroundColor: _isHovered
+              ? theme.colorScheme.onSurface.withValues(alpha: 0.06)
+              : Colors.transparent,
         ),
         child: Row(
           mainAxisAlignment: MainAxisAlignment.center,
@@ -140,7 +142,7 @@ class _TextLinkButtonState extends State<TextLinkButton> {
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
-    final effectiveColor = widget.color ?? AppColors.primaryColor;
+    final effectiveColor = widget.color ?? theme.colorScheme.primary;
 
     Widget button = AnimatedContainer(
       duration: const Duration(milliseconds: 200),