Admin-controlled public registration toggle

User wants the admin to be able to open or close public self-registration
from inside the app instead of removing the registration page entirely.

Changes:
- Added registration_open boolean column to app_settings (default true)
- Exposed registrationOpen in AppSettings + UpdateAppSettingsBody schemas
  in openapi.yaml; regenerated client + zod
- Backend: POST /api/auth/register now returns 403 "Registration is
  closed" when the flag is off (checks settings row before any other work)
- Admin Site Settings panel now has a switch to toggle public
  registration on/off, with bilingual label + helper text
- Login page hides the "Create account" link when registration is closed
- Register page short-circuits to a friendly "registration closed"
  card with a Back to Login button when the flag is off (and redirects
  if the user lands there directly)
- Added bilingual locale keys: registrationOpen, registrationOpenHint,
  registrationClosed

Verified: GET /api/settings returns the new field; POST /api/auth/register
returns 403 when closed and proceeds normally when open. Both typecheck
suites pass.

artifacts/api-server/src/routes/auth.ts

@@ -6,6 +6,7 @@ import {
   usersTable,
   userRolesTable,
   rolesTable,
+  appSettingsTable,
 } from "@workspace/db";
 import { requireAuth, getUserRoles } from "../middlewares/auth";
 import { RegisterBody, LoginBody, UpdateLanguageBody } from "@workspace/api-zod";
@@ -34,6 +35,12 @@ router.post("/auth/register", async (req, res): Promise<void> => {
     return;
   }
 
+  const [settings] = await db.select().from(appSettingsTable).where(eq(appSettingsTable.id, 1));
+  if (settings && !settings.registrationOpen) {
+    res.status(403).json({ error: "Registration is closed" });
+    return;
+  }
+
   const { username, email, password, displayNameAr, displayNameEn, preferredLanguage } = parsed.data;
 
   const existing = await db