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

.env.example

@@ -1,3 +1,3 @@
-# URL of the PocketBase backend (auth + database).
+# URL of the Directus backend (auth + database).
 # Falls back to the production instance if unset.
-POCKETBASE_URL=https://pokeshare.namarusaja.me
+DIRECTUS_URL=https://directus.namarusaja.me