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/entity-nav.svelte.spec.ts

@@ -13,6 +13,7 @@ const props = {
 	userCount: 5,
 	groupCount: 3,
 	tagCount: 8,
+	inviteCount: 2,
 	canManageUsers: true,
 	canManageTags: true,
 	canManagePermissions: true,