feat: Yandex Metrika CID tracking, offline conversions UI, sticky pay button

- Pass yandex_cid to all auth endpoints (telegram, email, OIDC, OAuth)
- Add OfflineConvGoal interface and offline_conv_* fields to AnalyticsCounters
- Add storeYandexCid API method for server-side CID persistence
- Add analytics fields to LandingConfig (view/click goals, sticky_pay_button)
- Add yandex_cid/referrer/subid to PurchaseRequest
- Add offline conversions UI block in admin AnalyticsTab
- Add cacheYandexCid, syncYandexCid, fireAnalyticsEvent to analytics hook
- Create yandexCid.ts utility (localStorage get/set helpers)
- Add sticky pay button with portal on mobile in QuickPurchase
- Fire view/click analytics goals on landing pages
- Persist contact value and referrer/subid in session/localStorage
- Add i18n keys for offlineConv and apiKey in all 4 locales

src/locales/zh.json

@@ -1709,7 +1709,9 @@
         "telegramOidcEnabled": "启用 OIDC",
         "telegramOidcClientId": "Client ID（机器人 ID）",
         "telegramOidcClientSecret": "Client Secret"
-      }
+      },
+      "offlineConv": "\u79bb\u7ebf\u8f6c\u5316",
+      "apiKey": "API \u5bc6\u94a5"
     },
     "buttons": {
       "color": "按钮颜色",