i18n(ai-chat): add the missing ru-RU typing-indicator keys (#109)

ru-RU had only '{{name}} is typing…' but not 'AI agent' / 'AI agent is typing…',
so the Russian typing indicator was mixed-language. Add them (AI-агент / AI-агент
печатает…) grouped with the named key. en-US is already complete; other locales
intentionally keep the en-US fallback (full translation is a separate effort).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/public/locales/ru-RU/translation.json

@@ -668,6 +668,8 @@
   "AI search": "Поиск ИИ",
   "AI Answer": "Ответ ИИ",
   "Ask AI": "Спросить ИИ",
+  "AI agent": "AI-агент",
+  "AI agent is typing…": "AI-агент печатает…",
   "{{name}} is typing…": "{{name}} печатает…",
   "AI is thinking...": "ИИ обрабатывает запрос...",
   "Thinking": "Думаю",