create admin_ui

Co-authored-by: Copilot <copilot@github.com>

admin_ui/index.html

@@ -0,0 +1,13 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Transit Admin</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script src="/env-config.js"></script>
+    <script type="module" src="/src/main.jsx"></script>
+  </body>
+</html>