feat: 进行功能分区方便后期维护

- 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/web/index.html

@@ -1,45 +1,44 @@
 <!DOCTYPE html>
-<html>
+<html lang="zh-CN">
 <head>
   <!--
-    If you are serving your web app in a path other than the root, change the
-    href value below to reflect the base path you are serving from.
-
-    The path provided below has to start and end with a slash "/" in order for
-    it to work correctly.
-
-    For more details:
-    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
-
-    This is a placeholder for base href that will be replaced by the value of
-    the `--base-href` argument provided to `flutter build`.
+    如果应用部署在子路径下，把下面的 href 改成对应前缀，
+    必须以 "/" 开头和结尾。构建时会由 flutter build 的 --base-href 替换。
   -->
   <base href="$FLUTTER_BASE_HREF">
 
   <meta charset="UTF-8">
   <meta content="IE=Edge" http-equiv="X-UA-Compatible">
-  <meta name="description" content="A new Flutter project.">
+  <meta name="description" content="Suxing Nav —— 极简导航站，把常用站点收进一个页面，随手可用。">
 
-  <!-- iOS meta tags & icons -->
+  <!-- 浏览器地址栏与移动端状态栏配色，与 AppColors.primary 保持一致 -->
+  <meta name="theme-color" content="#3B82F6">
+
+  <!-- iOS 添加到主屏幕时使用的元信息 -->
   <meta name="mobile-web-app-capable" content="yes">
   <meta name="apple-mobile-web-app-status-bar-style" content="black">
-  <meta name="apple-mobile-web-app-title" content="frontend">
+  <meta name="apple-mobile-web-app-title" content="Suxing Nav">
   <link rel="apple-touch-icon" href="icons/Icon-192.png">
 
   <!-- Favicon -->
   <link rel="icon" type="image/png" href="favicon.png"/>
 
-  <title>frontend</title>
+  <title>Suxing Nav</title>
   <link rel="manifest" href="manifest.json">
+
+  <style>
+    /* Flutter 引擎加载完成前 body 是空的，先把底色铺成 AppColors.background，
+       避免首屏闪一下白。 */
+    body {
+      margin: 0;
+      background-color: #F7F9FB;
+    }
+  </style>
 </head>
 <body>
   <!--
-    You can customize the "flutter_bootstrap.js" script.
-    This is useful to provide a custom configuration to the Flutter loader
-    or to give the user feedback during the initialization process.
-
-    For more details:
-    * https://docs.flutter.dev/platform-integration/web/initialization
+    flutter_bootstrap.js 可以自定义，用来给 Flutter 加载器传参，
+    或在初始化期间给用户反馈。详见 Flutter 官方 Web 初始化文档。
   -->
   <script src="flutter_bootstrap.js" async></script>
 </body>