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/components/LoginForm.astro

@@ -6,10 +6,10 @@
     <section>
         <form id="login-form" class="grid gap-4">
             <div role="group" class="field">
-                <label for="username">Nom d'utilisateur</label>
+                <label for="username">Email</label>
                 <input
                     class="input"
-                    type="text"
+                    type="email"
                     id="username"
                     name="username"
                     placeholder="votre@email.com"

src/components/Nav.astro

@@ -1,14 +1,8 @@
 ---
-const { pb } = Astro.locals;
-const user = pb.authStore.record;
-const initials = user
-    ? (user.name || user.email || "")
-          .split(/[\s@.]+/)
-          .filter(Boolean)
-          .slice(0, 2)
-          .map((s: string) => s[0].toUpperCase())
-          .join("")
-    : "";
+import { displayName, initials as userInitials } from "../lib/directus";
+
+const user = Astro.locals.user;
+const initials = user ? userInitials(user) : "";
 ---
 
 <header
@@ -130,7 +124,7 @@ const initials = user
                                     <span>{initials}</span>
                                 </span>
                                 <span class="hidden sm:inline ml-1 text-sm">
-                                    {user.name || user.email}
+                                    {displayName(user)}
                                 </span>
                                 <svg
                                     data-icon="inline-end"