feat(sprint1-step4): SuperOAuth login frontend

- AuthContext : fix exports, x-auth-token header, loginWithOAuth(), suppression axios/jwt-decode
- Login.jsx : redirect SuperOAuth Discord avec tenantId=clickerz
- AuthCallback.jsx : extraction token query param, flow OAuth complet
- .env.sample : ajout VITE_SUPEROAUTH_URL
- Mode invité préservé (pas de route guard)

Frontend/src/pages/AuthCallback.jsx

@@ -0,0 +1,46 @@
+import { useEffect, useState } from "react";
+import { useNavigate, useSearchParams, Link } from "react-router-dom";
+import { useAuth } from "../context/AuthContext";
+import "../scss/pages.scss";
+
+export default function AuthCallback() {
+  const [searchParams] = useSearchParams();
+  const { loginWithOAuth } = useAuth();
+  const navigate = useNavigate();
+  const [error, setError] = useState(null);
+
+  useEffect(() => {
+    const token = searchParams.get("token");
+
+    if (!token) {
+      setError("Token manquant dans l'URL.");
+      return;
+    }
+
+    loginWithOAuth(token)
+      .then(() => navigate("/", { replace: true }))
+      .catch((err) => setError(err.message || "Erreur de connexion."));
+  }, []);
+
+  if (error) {
+    return (
+      <section>
+        <div className="containererror">
+          <h1>Erreur de connexion</h1>
+          <p className="message">{error}</p>
+          <Link className="btn-return" to="/login">
+            Retour au login
+          </Link>
+        </div>
+      </section>
+    );
+  }
+
+  return (
+    <section>
+      <div className="containererror">
+        <p className="message">Connexion en cours...</p>
+      </div>
+    </section>
+  );
+}

Frontend/src/pages/Login.jsx

@@ -0,0 +1,32 @@
+import { useEffect } from "react";
+import { useNavigate } from "react-router-dom";
+import { useAuth } from "../context/AuthContext";
+import "../scss/pages.scss";
+
+const SUPEROAUTH_URL = import.meta.env.VITE_SUPEROAUTH_URL;
+
+export default function Login() {
+  const { user } = useAuth();
+  const navigate = useNavigate();
+
+  useEffect(() => {
+    if (user) navigate("/", { replace: true });
+  }, [user, navigate]);
+
+  const handleLogin = () => {
+    const callbackUrl = `${window.location.origin}/callback`;
+    window.location.href = `${SUPEROAUTH_URL}/api/v1/oauth/discord?redirectUrl=${encodeURIComponent(callbackUrl)}&tenantId=clickerz`;
+  };
+
+  return (
+    <section>
+      <div className="containererror">
+        <h1>Connexion</h1>
+        <p className="message">Connecte-toi pour sauvegarder ta progression.</p>
+        <button className="btn-return" onClick={handleLogin} type="button">
+          Se connecter avec SuperOAuth
+        </button>
+      </div>
+    </section>
+  );
+}