fix: login page avec sélection provider → /api/v1/auth/oauth/:provider?redirectUrl

SuperOAuth root page ignore ?redirectUrl, les boutons de sa UI pointent vers
ses propres URIs hardcodées. Fix : notre page /login construit les URLs API
directement avec redirectUrl=origins.tetardtek.com/callback.

Aussi : CORS_ORIGINS SuperOAuth mis à jour (origins.tetardtek.com ajouté).

frontend/src/pages/LoginPage.tsx

@@ -0,0 +1,39 @@
+import { Link } from 'react-router-dom';
+
+const PROVIDERS = [
+  { id: 'discord', label: 'Discord' },
+  { id: 'github',  label: 'GitHub'  },
+  { id: 'google',  label: 'Google'  },
+  { id: 'twitch',  label: 'Twitch'  },
+] as const;
+
+export default function LoginPage() {
+  const base = import.meta.env.VITE_SUPEROAUTH_URL;
+  const redirectUrl = encodeURIComponent(window.location.origin + '/callback');
+
+  return (
+    <div className="flex flex-col items-center gap-8 pt-20">
+      <div className="flex flex-col items-center gap-2">
+        <span className="font-mono text-xs font-bold tracking-widest text-od-accent">OD</span>
+        <h1 className="text-xl font-semibold text-od-text">Connexion</h1>
+        <p className="text-sm text-od-muted">Choisis ton provider pour continuer</p>
+      </div>
+
+      <div className="flex w-full max-w-xs flex-col gap-3">
+        {PROVIDERS.map(({ id, label }) => (
+          <a
+            key={id}
+            href={`${base}/api/v1/auth/oauth/${id}?redirectUrl=${redirectUrl}`}
+            className="flex items-center justify-center rounded border border-od-border bg-od-surface px-4 py-3 text-sm text-od-text transition-colors hover:border-od-accent hover:text-od-accent"
+          >
+            {label}
+          </a>
+        ))}
+      </div>
+
+      <Link to="/" className="font-mono text-xs text-od-muted hover:text-od-text transition-colors">
+        ← Retour
+      </Link>
+    </div>
+  );
+}