feat: 实现导航页基础功能

- 添加导航条目和分类数据模型
- 实现导航卡片网格展示
- 支持点击卡片打开链接
- 添加主题样式（渐变色、卡片阴影）
- 区分系统默认和自定义条目
- 添加分类统计信息"

frontend/lib/theme/app_theme.dart

@@ -0,0 +1,54 @@
+import 'package:flutter/material.dart';
+
+class AppTheme {
+  // 颜色方案
+  static const Color primaryColor = Color(0xFF2563EB);
+  static const Color secondaryColor = Color(0xFF7C3AED);
+  static const Color backgroundColor = Color(0xFFF8FAFC);
+  static const Color cardColor = Colors.white;
+  static const Color systemTagColor = Color(0xFFE2E8F0);
+  static const Color systemTagTextColor = Color(0xFF64748B);
+
+  // 渐变背景
+  static final LinearGradient appBarGradient = LinearGradient(
+    colors: [primaryColor, secondaryColor],
+    begin: Alignment.centerLeft,
+    end: Alignment.centerRight,
+  );
+
+  // 卡片阴影
+  static final List<BoxShadow> cardShadow = [
+    BoxShadow(
+      color: Colors.grey.withOpacity(0.1),
+      spreadRadius: 2,
+      blurRadius: 8,
+      offset: const Offset(0, 2),
+    ),
+  ];
+
+  // 主题数据
+  static ThemeData lightTheme = ThemeData(
+    useMaterial3: true,
+    colorScheme: ColorScheme.fromSeed(
+      seedColor: primaryColor,
+      primary: primaryColor,
+      secondary: secondaryColor,
+      background: backgroundColor,
+    ),
+    scaffoldBackgroundColor: backgroundColor,
+    cardTheme: CardThemeData(
+      elevation: 0,
+      shape: RoundedRectangleBorder(
+        borderRadius: BorderRadius.circular(16),
+      ),
+      color: cardColor,
+      shadowColor: Colors.transparent,
+    ),
+    appBarTheme: const AppBarTheme(
+      elevation: 0,
+      centerTitle: true,
+      backgroundColor: Colors.transparent,
+      foregroundColor: Colors.white,
+    ),
+  );
+}