feat: Add promo preference survey on main page

- Add promo preference fields to User model (promo_preferred_maker,
  promo_preferred_model, promo_email_enabled)
- Create API endpoints for getting/updating promo preferences
- Create PromoPreference component with maker/model selection
- Show login prompt for non-logged-in users when interacting
- Add promo notification service to send emails when matching vehicles
  are added to promotion
- Add multi-language translations (en, mn, ru, ko)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/page.tsx

@@ -3,6 +3,7 @@
 import { useState, useEffect } from 'react';
 import Link from 'next/link';
 import FilmStripSlider from '@/components/FilmStripSlider';
+import PromoPreference from '@/components/PromoPreference';
 import { HeroBanner, HeroBannerSettings } from '@/types';
 import { heroBannersApi } from '@/lib/api';
 import { useTranslation } from '@/lib/i18n';
@@ -45,13 +46,23 @@ export default function Home() {
         {/* Film Strip Slider */}
         <FilmStripSlider banners={banners} settings={bannerSettings} />
 
-        <div className="container mx-auto px-4 py-4 sm:py-8 text-center">
-          <Link
-            href="/vehicle-request"
-            className="inline-block bg-yellow-500 text-white font-semibold px-6 py-2.5 sm:px-8 sm:py-3 rounded-lg hover:bg-yellow-600 transition shadow-lg text-sm sm:text-base"
-          >
-            {t.requestVehicle}
-          </Link>
+        <div className="container mx-auto px-4 py-4 sm:py-8">
+          <div className="flex flex-col lg:flex-row items-center justify-center gap-4 lg:gap-8">
+            {/* Request Vehicle Button */}
+            <Link
+              href="/vehicle-request"
+              className="inline-block bg-yellow-500 text-white font-semibold px-6 py-2.5 sm:px-8 sm:py-3 rounded-lg hover:bg-yellow-600 transition shadow-lg text-sm sm:text-base"
+            >
+              {t.requestVehicle}
+            </Link>
+
+            {/* Divider */}
+            <div className="hidden lg:block w-px h-24 bg-white/30"></div>
+            <div className="lg:hidden w-32 h-px bg-white/30"></div>
+
+            {/* Promo Preference */}
+            <PromoPreference />
+          </div>
         </div>
       </section>