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/hooks.server.ts

@@ -0,0 +1,21 @@
+import type { Handle } from '@sveltejs/kit';
+import { decodeSession, SESSION_COOKIE } from '$lib/server/auth';
+
+export const handle: Handle = async ({ event, resolve }) => {
+	const session = await decodeSession(event.cookies.get(SESSION_COOKIE));
+
+	if (session && session.expiresAt * 1000 > Date.now()) {
+		event.locals.user = {
+			sub: session.sub,
+			name: session.name,
+			email: session.email,
+			picture: session.picture
+		};
+		event.locals.accessToken = session.accessToken;
+	} else {
+		event.locals.user = null;
+		event.locals.accessToken = null;
+	}
+
+	return resolve(event);
+};