i18n(ai-chat): complete the ru-RU AI-chat strings + record locale policy (#109)

ru-RU was missing most AI-chat keys, so the chat/typing widgets rendered
mixed-language (some keys fell back to en-US). Fill the full AI-chat string
set in ru-RU and document the maintenance policy.

- ru-RU/translation.json: add the 24 missing AI-chat keys (labels, typing
  indicator, Ask-AI widget, public-share, error messages); keep the typing
  keys grouped; existing translations untouched.
- i18n.ts: add a policy comment near fallbackLng — en-US is the source of
  truth; en-US + ru-RU are fully maintained; the other 10 locales
  intentionally rely on the en-US fallback until contributed.

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

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

@@ -671,6 +671,30 @@
   "AI agent": "AI-агент",
   "AI agent is typing…": "AI-агент печатает…",
   "{{name}} is typing…": "{{name}} печатает…",
+  "Agent role": "Роль агента",
+  "AI chat": "AI-чат",
+  "AI chat is disabled for this workspace.": "AI-чат отключён для этого рабочего пространства.",
+  "Ask a question about this documentation.": "Задайте вопрос об этой документации.",
+  "Ask a question…": "Задайте вопрос…",
+  "Ask the AI agent anything about your workspace.": "Спросите AI-агента о чём угодно по вашему рабочему пространству.",
+  "Ask the AI agent…": "Спросите AI-агента…",
+  "Copy chat": "Копировать чат",
+  "Created successfully": "Успешно создано",
+  "Current context size": "Текущий размер контекста",
+  "Delete this chat?": "Удалить этот чат?",
+  "Deleted successfully": "Успешно удалено",
+  "Edited by AI agent on behalf of {{name}}": "Отредактировано AI-агентом от имени {{name}}",
+  "Failed to delete chat": "Не удалось удалить чат",
+  "Failed to rename chat": "Не удалось переименовать чат",
+  "Minimize": "Свернуть",
+  "No chats yet.": "Чатов пока нет.",
+  "Send": "Отправить",
+  "Something went wrong": "Что-то пошло не так",
+  "Stop": "Стоп",
+  "The AI agent could not respond. Please try again.": "AI-агент не смог ответить. Попробуйте ещё раз.",
+  "The AI provider is not configured. Ask an administrator to set it up.": "AI-провайдер не настроен. Попросите администратора настроить его.",
+  "Universal assistant": "Универсальный ассистент",
+  "You": "Вы",
   "AI is thinking...": "ИИ обрабатывает запрос...",
   "Thinking": "Думаю",
   "Ask a question...": "Задайте вопрос...",

apps/client/src/i18n.ts

@@ -12,6 +12,15 @@ i18n
   // init i18next
   // for all options read: https://www.i18next.com/overview/configuration-options
   .init({
+    // i18n maintenance policy:
+    // - en-US is the source of truth for all UI strings (keys are the English text).
+    // - en-US and ru-RU are the fully-maintained locales; in particular, the
+    //   AI-chat string set is kept complete in both so the UI never renders
+    //   mixed-language (no per-key en-US fallback within a single widget).
+    // - The other 10 locales (fr-FR, de-DE, es-ES, nl-NL, ja-JP, zh-CN, ko-KR,
+    //   pt-BR, it-IT, uk-UA) are partial and intentionally rely on the
+    //   `fallbackLng: "en-US"` fallback below until translations are
+    //   contributed (e.g. via Crowdin).
     fallbackLng: "en-US",
     debug: false,
     showSupportNotice: false,