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/app.d.ts

@@ -1,9 +1,14 @@
 // See https://svelte.dev/docs/kit/types#app.d.ts
 // for information about these interfaces
+import type { SessionUser } from '$lib/types';
+
 declare global {
 	namespace App {
 		// interface Error {}
-		// interface Locals {}
+		interface Locals {
+			user: SessionUser | null;
+			accessToken: string | null;
+		}
 		// interface PageData {}
 		// interface PageState {}
 		// interface Platform {}