chore: 将前端移动到 client/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/index.html

@@ -0,0 +1,24 @@
+<!doctype html>
+<html lang="zh-Hans-CN">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="shortcut icon" href="/noto--honeybee.ico" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>前端开发学习平台</title>
+    <link rel="stylesheet" href="/normalize.min.css" />
+    <link rel="stylesheet" href="/style.css" />
+    <script>
+      window.localStorage.setItem("maxkbMaskTip", true)
+    </script>
+    <% if (process.env.PUBLIC_MAXKB_URL) { %>
+    <script
+      async
+      defer
+      src="<%= process.env.PUBLIC_MAXKB_URL %>"
+    ></script>
+    <% } %>
+  </head>
+  <body>
+    <div id="app"></div>
+  </body>
+</html>