refactor: 使用 Clean Code 原则优化 Flutter 代码

- 替换硬编码数字为 AppSpacing/AppRadius 常量
- 提升代码可读性和可维护性
- 添加必要注释说明固定尺寸的原因

优化文件:
- home_page.dart: 替换魔法数字为间距系统常量
- main_page.dart: 统一使用间距系统常量

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

flutter_monisuo/lib/ui/pages/main/main_page.dart

@@ -2,6 +2,7 @@ import 'dart:ui';
 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 '../home/home_page.dart';
 import '../market/market_page.dart';
@@ -84,22 +85,22 @@ class _TopAppBar extends StatelessWidget {
     final isDark = Theme.of(context).brightness == Brightness.dark;
 
     return Container(
-      height: 64,
+      height: 64, // 顶部导航栏高度保持固定
       decoration: BoxDecoration(
         color: colorScheme.surfaceBright.withOpacity(0.4),
         boxShadow: [
           BoxShadow(
             color: colorScheme.primary.withOpacity(isDark ? 0.15 : 0.08),
-            blurRadius: 64,
+            blurRadius: 64, // 大模糊效果保持固定
             offset: const Offset(0, 32),
           ),
         ],
       ),
       child: ClipRRect(
         child: BackdropFilter(
-          filter: ImageFilter.blur(sigmaX: 24, sigmaY: 24),
+          filter: ImageFilter.blur(sigmaX: AppSpacing.xl, sigmaY: AppSpacing.xl),
           child: Padding(
-            padding: const EdgeInsets.symmetric(horizontal: 24),
+            padding: EdgeInsets.symmetric(horizontal: AppSpacing.lg),
             child: Row(
               mainAxisAlignment: MainAxisAlignment.spaceBetween,
               children: [
@@ -110,7 +111,7 @@ class _TopAppBar extends StatelessWidget {
                       children: [
                         // 头像
                         Container(
-                          width: 32,
+                          width: 32, // 头像尺寸保持固定
                           height: 32,
                           decoration: BoxDecoration(
                             shape: BoxShape.circle,
@@ -130,7 +131,7 @@ class _TopAppBar extends StatelessWidget {
                             ),
                           ),
                         ),
-                        const SizedBox(width: 12),
+                        SizedBox(width: AppSpacing.sm + AppSpacing.xs),
                         // 标题
                         Text(
                           '模拟所',
@@ -152,7 +153,7 @@ class _TopAppBar extends StatelessWidget {
                       icon: LucideIcons.search,
                       onTap: () {},
                     ),
-                    const SizedBox(width: 8),
+                    SizedBox(width: AppSpacing.sm),
                     _IconBtn(
                       icon: LucideIcons.bell,
                       onTap: () {},
@@ -182,7 +183,7 @@ class _IconBtn extends StatelessWidget {
     return GestureDetector(
       onTap: onTap,
       child: Container(
-        padding: const EdgeInsets.all(8),
+        padding: EdgeInsets.all(AppSpacing.sm),
         child: Icon(
           icon,
           color: colorScheme.onSurfaceVariant,
@@ -212,7 +213,7 @@ class _BottomNavBar extends StatelessWidget {
     return Container(
       decoration: BoxDecoration(
         color: colorScheme.surfaceContainerLow.withOpacity(0.8),
-        borderRadius: const BorderRadius.vertical(top: Radius.circular(32)),
+        borderRadius: BorderRadius.vertical(top: Radius.circular(AppRadius.xxl + AppSpacing.sm)),
         border: Border(
           top: BorderSide(
             color: colorScheme.outlineVariant.withOpacity(0.15),
@@ -221,18 +222,18 @@ class _BottomNavBar extends StatelessWidget {
         boxShadow: [
           BoxShadow(
             color: Colors.black.withOpacity(0.5),
-            blurRadius: 40,
+            blurRadius: 40, // 阴影效果保持固定
             offset: const Offset(0, -10),
           ),
         ],
       ),
       child: ClipRRect(
-        borderRadius: const BorderRadius.vertical(top: Radius.circular(32)),
+        borderRadius: BorderRadius.vertical(top: Radius.circular(AppRadius.xxl + AppSpacing.sm)),
         child: BackdropFilter(
-          filter: ImageFilter.blur(sigmaX: 16, sigmaY: 16),
+          filter: ImageFilter.blur(sigmaX: AppSpacing.md, sigmaY: AppSpacing.md),
           child: SafeArea(
             child: Padding(
-              padding: const EdgeInsets.fromLTRB(16, 8, 16, 24),
+              padding: EdgeInsets.fromLTRB(AppSpacing.md, AppSpacing.sm, AppSpacing.md, AppSpacing.lg),
               child: Row(
                 mainAxisAlignment: MainAxisAlignment.spaceAround,
                 children: items.asMap().entries.map((entry) {
@@ -273,15 +274,15 @@ class _NavItemWidget extends StatelessWidget {
       behavior: HitTestBehavior.opaque,
       child: AnimatedContainer(
         duration: const Duration(milliseconds: 300),
-        padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+        padding: EdgeInsets.symmetric(horizontal: AppSpacing.md, vertical: AppSpacing.sm),
         decoration: isSelected
             ? BoxDecoration(
                 color: colorScheme.primary.withOpacity(0.1),
-                borderRadius: BorderRadius.circular(16),
+                borderRadius: BorderRadius.circular(AppSpacing.md),
                 boxShadow: [
                   BoxShadow(
                     color: colorScheme.primary.withOpacity(0.3),
-                    blurRadius: 15,
+                    blurRadius: 15, // 发光效果保持固定
                     spreadRadius: 0,
                   ),
                 ],
@@ -295,7 +296,7 @@ class _NavItemWidget extends StatelessWidget {
               color: color,
               size: 24,
             ),
-            const SizedBox(height: 4),
+            SizedBox(height: AppSpacing.xs),
             Text(
               item.label,
               style: TextStyle(