refactor: update Russian locale to match new i18n system

- Create lib/i18n/locales/ru-RU.json with full Russian translation (878 keys)
- Register ru-RU in lib/i18n/locales.ts (code: 'ru-RU', label: 'Русский', shortLabel: 'RU')
- No AI prompt modifications — follows new i18next-based architecture

Author: maximvalerevich

lib/i18n/locales.ts

@@ -17,4 +17,5 @@ export const supportedLocales = [
   { code: 'zh-CN', label: '简体中文', shortLabel: 'CN' },
   { code: 'en-US', label: 'English', shortLabel: 'EN' },
   { code: 'ja-JP', label: '日本語', shortLabel: 'JA' },
+  { code: 'ru-RU', label: 'Русский', shortLabel: 'RU' },
 ] as const satisfies readonly LocaleEntry[];