feat(ui): 优化所有页面使用现代设计系统

- 使用 AppSpacing 替换硬编码间距
- 使用 AppRadius 替换硬编码圆角
- 使用 withValues(alpha:) 替换 withOpacity
- 优化 login_page, home_page, market_page, trade_page, asset_page, mine_page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flutter_monisuo/lib/ui/pages/auth/login_page.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:shadcn_ui/shadcn_ui.dart';
 import 'package:provider/provider.dart';
 
+import '../../../core/theme/app_spacing.dart';
 import '../../../providers/auth_provider.dart';
 import '../main/main_page.dart';
 import 'register_page.dart';
@@ -29,7 +30,7 @@ class _LoginPageState extends State<LoginPage> {
         child: ConstrainedBox(
           constraints: const BoxConstraints(maxWidth: _maxFormWidth),
           child: Padding(
-            padding: const EdgeInsets.all(24),
+            padding: EdgeInsets.all(AppSpacing.lg),
             child: ShadForm(
               key: formKey,
               child: Column(
@@ -37,13 +38,13 @@ class _LoginPageState extends State<LoginPage> {
                 crossAxisAlignment: CrossAxisAlignment.stretch,
                 children: [
                   _buildHeader(theme),
-                  const SizedBox(height: 48),
+                  SizedBox(height: AppSpacing.xxl),
                   _buildUsernameField(),
-                  const SizedBox(height: 16),
+                  SizedBox(height: AppSpacing.md),
                   _buildPasswordField(),
-                  const SizedBox(height: 24),
+                  SizedBox(height: AppSpacing.lg),
                   _buildLoginButton(),
-                  const SizedBox(height: 16),
+                  SizedBox(height: AppSpacing.md),
                   _buildRegisterLink(theme),
                 ],
               ),
@@ -62,13 +63,13 @@ class _LoginPageState extends State<LoginPage> {
           size: _logoSize,
           color: theme.colorScheme.primary,
         ),
-        const SizedBox(height: 24),
+        SizedBox(height: AppSpacing.lg),
         Text(
           '模拟所',
           style: theme.textTheme.h1,
           textAlign: TextAlign.center,
         ),
-        const SizedBox(height: 8),
+        SizedBox(height: AppSpacing.sm),
         Text(
           '虚拟货币模拟交易平台',
           style: theme.textTheme.muted,