feat(frontend): invite-based registration UI

- Add /register route with invite code prefill, password show/hide
- Add /login?registered=1 success banner
- Add /admin/invites page: list, create, revoke, copy link
- Add Einladungen nav section to admin sidebar (ADMIN_USER perm)
- Add invite error codes to errors.ts
- Add 48 i18n keys across de/en/es
- Update hooks.server.ts to allow public access to invite/register API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/admin/+layout.svelte

@@ -19,6 +19,7 @@ let { data, children } = $props();
 			userCount={data.userCount}
 			groupCount={data.groupCount}
 			tagCount={data.tagCount}
+			inviteCount={data.inviteCount}
 			canManageUsers={data.canManageUsers}
 			canManageTags={data.canManageTags}
 			canManagePermissions={data.canManagePermissions}