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/lib/api.ts

@@ -1,5 +1,5 @@
 import axios from 'axios';
-import { Car, CarListResponse, CarMaker, CarModel, User, HeroBanner, HeroBannerSettings, CarView } from '@/types';
+import { Car, CarListResponse, CarMaker, CarModel, User, HeroBanner, HeroBannerSettings, CarView, PromoPreference } from '@/types';
 
 // When NEXT_PUBLIC_API_URL is empty, use relative path (for HTTPS proxy setup)
 const API_URL = process.env.NEXT_PUBLIC_API_URL;
@@ -133,6 +133,18 @@ export const authApi = {
     const { data } = await api.post('/auth/withdraw/cancel');
     return data;
   },
+
+  // 프로모션 선호 차종 조회
+  getPromoPreference: async (): Promise<PromoPreference> => {
+    const { data } = await api.get('/auth/me/promo-preference');
+    return data;
+  },
+
+  // 프로모션 선호 차종 설정
+  updatePromoPreference: async (preference: PromoPreference): Promise<PromoPreference> => {
+    const { data } = await api.put('/auth/me/promo-preference', preference);
+    return data;
+  },
 };
 
 // Inquiries API

frontend/src/lib/i18n.ts

@@ -121,6 +121,17 @@ export interface Translations {
   contactForPrice: string;
   engine: string;
 
+  // Promo Preference
+  promoPreferenceTitle: string;
+  promoPreferenceDesc: string;
+  selectMaker: string;
+  selectModel: string;
+  promoEmailAlert: string;
+  promoLoginRequired: string;
+  savePreference: string;
+  saving: string;
+  saved: string;
+
   // Vehicle Request & CC System
   requestVehicle: string;
   aiDealerSearching: string;
@@ -592,6 +603,17 @@ const translations: Record<Language, Translations> = {
     contactForPrice: 'Contact for price',
     engine: 'Engine',
 
+    // Promo Preference
+    promoPreferenceTitle: 'Next Promotion Interest',
+    promoPreferenceDesc: 'Tell us which vehicle you want in the next promotion!',
+    selectMaker: 'Select Maker',
+    selectModel: 'Select Model',
+    promoEmailAlert: 'Notify me by email when this vehicle is on promotion',
+    promoLoginRequired: 'Please login to save your preference',
+    savePreference: 'Save Preference',
+    saving: 'Saving...',
+    saved: 'Saved!',
+
     // Vehicle Request & CC System
     requestVehicle: 'Request Vehicle',
     aiDealerSearching: 'Korean AI Dealer is searching for vehicles...',
@@ -1061,6 +1083,17 @@ const translations: Record<Language, Translations> = {
     contactForPrice: 'Үнийг лавлана уу',
     engine: 'Хөдөлгүүр',
 
+    // Promo Preference
+    promoPreferenceTitle: 'Дараагийн урамшуулалд сонирхолтой',
+    promoPreferenceDesc: 'Дараагийн урамшуулалд ямар машин хүсч байгаагаа хэлнэ үү!',
+    selectMaker: 'Үйлдвэрлэгч сонгох',
+    selectModel: 'Загвар сонгох',
+    promoEmailAlert: 'Энэ машин урамшуулалд гарахад имэйлээр мэдэгдэнэ үү',
+    promoLoginRequired: 'Сонголтоо хадгалахын тулд нэвтэрнэ үү',
+    savePreference: 'Сонголт хадгалах',
+    saving: 'Хадгалж байна...',
+    saved: 'Хадгалагдлаа!',
+
     // Vehicle Request & CC System
     requestVehicle: 'Машин захиалах',
     aiDealerSearching: 'Солонгосын AI дилер машин хайж байна...',
@@ -1530,6 +1563,17 @@ const translations: Record<Language, Translations> = {
     contactForPrice: 'Цена по запросу',
     engine: 'Двигатель',
 
+    // Promo Preference
+    promoPreferenceTitle: 'Интерес к следующей акции',
+    promoPreferenceDesc: 'Расскажите, какой автомобиль вы хотите в следующей акции!',
+    selectMaker: 'Выберите марку',
+    selectModel: 'Выберите модель',
+    promoEmailAlert: 'Уведомить по email когда этот автомобиль появится в акции',
+    promoLoginRequired: 'Войдите, чтобы сохранить предпочтения',
+    savePreference: 'Сохранить предпочтения',
+    saving: 'Сохранение...',
+    saved: 'Сохранено!',
+
     // Vehicle Request & CC System
     requestVehicle: 'Запросить автомобиль',
     aiDealerSearching: 'Корейский AI-дилер ищет автомобили...',
@@ -1999,6 +2043,17 @@ const translations: Record<Language, Translations> = {
     contactForPrice: '가격 문의',
     engine: '엔진',
 
+    // Promo Preference
+    promoPreferenceTitle: '다음 프로모션 관심 차종',
+    promoPreferenceDesc: '다음 프로모션에 원하는 차량을 알려주세요!',
+    selectMaker: '제조사 선택',
+    selectModel: '모델 선택',
+    promoEmailAlert: '이 차량이 프로모션에 등록되면 이메일로 알림받기',
+    promoLoginRequired: '선호도를 저장하려면 로그인하세요',
+    savePreference: '선호도 저장',
+    saving: '저장 중...',
+    saved: '저장됨!',
+
     // Vehicle Request & CC System
     requestVehicle: '차량요청하기',
     aiDealerSearching: '한국AI딜러가 차량을 찾고있습니다...',