feat(frontend): server-side Zitadel OIDC auth (part 2)

Add Authorization Code + PKCE login/logout against Zitadel, handled entirely
server-side with an httpOnly signed session cookie:
- $lib/server/auth: openid-client discovery (config cached), jose-signed
  session encode/decode, cookie/name constants
- routes: /login (builds the authorize URL with PKCE/state/nonce, ?mode=register
  -> prompt=create), /auth/callback (code exchange + session cookie),
  /logout (clears session + Zitadel end_session)
- hooks.server populates locals.user/accessToken from the session
- +layout.server exposes the user; Navbar shows the signed-in user + Salir,
  or Iniciar sesión / Registrarse
- switch to adapter-node; env-driven config (.env gitignored, .env.example
  committed) with issuer http://host.docker.internal:8080

Verified: svelte-check + build clean; dev smoke test shows the landing in the
logged-out state and /login issuing a 302 to Zitadel's authorize endpoint with
PKCE/state/nonce (discovery reachable). Real client id/secret still needed to
complete the token exchange.

frontend/src/routes/auth/callback/+server.ts

@@ -0,0 +1,69 @@
+import { redirect, error } from '@sveltejs/kit';
+import * as client from 'openid-client';
+import type { RequestHandler } from './$types';
+import {
+	getOidcConfig,
+	encodeSession,
+	type Session,
+	SESSION_COOKIE,
+	STATE_COOKIE,
+	VERIFIER_COOKIE,
+	NONCE_COOKIE,
+	RETURN_COOKIE
+} from '$lib/server/auth';
+
+export const GET: RequestHandler = async ({ url, cookies }) => {
+	const codeVerifier = cookies.get(VERIFIER_COOKIE);
+	const state = cookies.get(STATE_COOKIE);
+	const nonce = cookies.get(NONCE_COOKIE);
+	const returnTo = cookies.get(RETURN_COOKIE) ?? '/explore';
+
+	if (!codeVerifier || !state) {
+		error(400, 'Estado de autenticación inválido. Vuelve a iniciar sesión.');
+	}
+
+	const config = await getOidcConfig();
+
+	let tokens;
+	try {
+		tokens = await client.authorizationCodeGrant(config, url, {
+			pkceCodeVerifier: codeVerifier,
+			expectedState: state,
+			expectedNonce: nonce,
+			idTokenExpected: true
+		});
+	} catch (exception) {
+		console.error('Token exchange failed', exception);
+		error(401, 'La autenticación ha fallado.');
+	}
+
+	const claims = tokens.claims();
+	if (!claims) {
+		error(401, 'El proveedor no devolvió el token de identidad.');
+	}
+
+	const session: Session = {
+		sub: String(claims.sub),
+		name: claims.name as string | undefined,
+		email: claims.email as string | undefined,
+		picture: claims.picture as string | undefined,
+		accessToken: tokens.access_token,
+		refreshToken: tokens.refresh_token,
+		idToken: tokens.id_token,
+		expiresAt: Math.floor(Date.now() / 1000) + (tokens.expires_in ?? 3600)
+	};
+
+	cookies.set(SESSION_COOKIE, await encodeSession(session), {
+		httpOnly: true,
+		sameSite: 'lax',
+		secure: url.protocol === 'https:',
+		path: '/',
+		maxAge: 60 * 60 * 24 * 7
+	});
+
+	for (const name of [STATE_COOKIE, VERIFIER_COOKIE, NONCE_COOKIE, RETURN_COOKIE]) {
+		cookies.delete(name, { path: '/' });
+	}
+
+	redirect(302, returnTo);
+};