Sync React frontend into git

- Replace stale TanStack Router scaffold with working react-router-dom app
- Add all 9 page components + cyber theme CSS
- Remove routeTree.gen.ts, add index.css, query.ts

frontend/src/components/LoginPage.tsx

@@ -0,0 +1,23 @@
+import { useState } from "react";
+import { useMutation } from "@tanstack/react-query";
+import { api } from "../lib/api";
+
+export default function LoginPage() {
+  const [mailbox, setMailbox] = useState("");
+  const [pin, setPin] = useState("");
+  const [error, setError] = useState("");
+  const login = useMutation({ mutationFn: () => api.login(mailbox, pin) });
+  const submit = (e: React.FormEvent) => { e.preventDefault(); setError(""); login.mutate(undefined, { onSuccess: () => { window.location.href = "/"; }, onError: (err) => setError((err as Error).message) }); };
+  return (
+    <div className="wrap">
+      <div className="card">
+        {error && <div className="err">{error}</div>}
+        <form onSubmit={submit}>
+          <label>Mailbox number</label><input value={mailbox} onChange={(e) => setMailbox(e.target.value)} inputMode="numeric" />
+          <label>PIN</label><input type="password" value={pin} onChange={(e) => setPin(e.target.value)} inputMode="numeric" />
+          <div className="row"><button type="submit" className="primary">Log in</button></div>
+        </form>
+      </div>
+    </div>
+  );
+}