fix(ai-chat): always show generic "AI is typing…" indicator

The typing indicator rendered "<role name> is typing…". Show a generic
"AI is typing…" instead and keep the role/identity name only in the
dimmed interlocutor label above the typing dots.

- typing line now always renders t("AI is typing…")
- add the "AI is typing…" key to en-US and ru-RU locales
- sync stale doc comments that referenced the old text

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

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

@@ -672,6 +672,7 @@
   "Take a look at the current document": "Посмотри текущий документ",
   "AI agent is typing…": "AI-агент печатает…",
   "{{name}} is typing…": "{{name}} печатает…",
+  "AI is typing…": "AI печатает…",
   "Agent role": "Роль агента",
   "AI chat": "AI-чат",
   "AI chat is disabled for this workspace.": "AI-чат отключён для этого рабочего пространства.",