Migrate backend from PocketBase to Directus

Full cutover: auth, users, collection and friendships now run on Directus
via @directus/sdk, with owner/visibility rules enforced by Directus
permissions.

- middleware: Directus session (directus_auth cookie) with token refresh
- /login is Directus (email normalized to lowercase), new /register page
  (first/last name, username) backed by public registration
- collection, friends and profile pages + all API endpoints ported;
  ownership and addressee-only accept/decline enforced by permissions
- display names use first_name/last_name everywhere (nav, friends, profile)
- PocketBase SDK, POCKETBASE_URL and pb_schema.json removed
- README and compose updated for DIRECTUS_URL

src/pages/search.astro

@@ -2,9 +2,11 @@
 import BaseLayout from "../layouts/BaseLayout.astro";
 import { getSets } from "../lib/tcgdex-cache";
 
+import { isLoggedIn } from "../lib/auth";
+
 const { pb } = Astro.locals;
 
-if (!pb.authStore.isValid) {
+if (!isLoggedIn(Astro.locals)) {
     return Astro.redirect("/login");
 }