fix: restore frontend source code (src/web/) that was accidentally deleted

Frontend source is needed for development/building the SPA.
Only Node.js runtime server code should be removed.

src/web/index.html

@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<html lang="zh-CN">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+  <meta name="description" content="BoosAPI - 中转站的中转站，聚合多个 AI API 中转站为统一网关" />
+  <title>BoosAPI</title>
+  <link rel="icon" type="image/png" href="/favicon.png" />
+  <link rel="preconnect" href="https://fonts.googleapis.com" />
+  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
+  <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap" rel="stylesheet" />
+</head>
+<body class="min-h-screen">
+  <script>
+    (function(){
+      var t = localStorage.getItem('theme');
+      if (t === 'dark' || (!t && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
+        document.documentElement.setAttribute('data-theme', 'dark');
+      } else {
+        document.documentElement.setAttribute('data-theme', 'light');
+      }
+    })();
+  </script>
+  <div id="root"></div>
+  <script type="module" src="./main.tsx"></script>
+</body>
+</html>