feat(auth): add frontend login and route protection

frontend/athena/app/login/page.tsx

@@ -0,0 +1,79 @@
+"use client";
+
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+
+export default function LoginPage() {
+  const router = useRouter();
+
+  const [username, setUsername] = useState("");
+  const [password, setPassword] = useState("");
+  const [error, setError] = useState("");
+
+  async function handleLogin(e: React.FormEvent) {
+    e.preventDefault();
+    setError("");
+
+    const response = await fetch(
+      `${process.env.NEXT_PUBLIC_API_URL}/auth/login`,
+      {
+        method: "POST",
+        credentials: "include",
+        headers: {
+          "Content-Type": "application/json",
+        },
+        body: JSON.stringify({
+          username,
+          password,
+        }),
+      }
+    );
+
+    if (response.ok) {
+      router.push("/dashboard");
+      return;
+    }
+
+    setError("Benutzername oder Passwort ist falsch.");
+  }
+
+  return (
+    <main className="flex min-h-screen items-center justify-center bg-slate-100">
+      <form
+        onSubmit={handleLogin}
+        className="w-full max-w-md rounded-xl bg-white p-8 shadow-xl"
+      >
+        <h1 className="mb-8 text-center text-3xl font-bold">
+          Olympus CRM
+        </h1>
+
+        <input
+          className="mb-4 w-full rounded border p-3"
+          placeholder="Benutzername"
+          value={username}
+          onChange={(e) => setUsername(e.target.value)}
+        />
+
+        <input
+          className="mb-4 w-full rounded border p-3"
+          type="password"
+          placeholder="Passwort"
+          value={password}
+          onChange={(e) => setPassword(e.target.value)}
+        />
+
+        {error && (
+          <p className="mb-4 text-red-600">
+            {error}
+          </p>
+        )}
+
+        <button
+          className="w-full rounded bg-blue-600 p-3 text-white hover:bg-blue-700"
+        >
+          Anmelden
+        </button>
+      </form>
+    </main>
+  );
+}