feat(ai-chat): role cards start the chat and show role identity

Rework the new-chat role-card empty state:
- Remove the "Universal assistant" card; universal assistant is now the
  implicit default the user gets by typing without picking a card.
- Show each role's description on its card (under the emoji and name).
- Clicking a card immediately starts the chat: it binds the role to the
  new chat and sends the default opening prompt "Take a look at the
  current document" (one click, no separate select step). roleIdRef is
  set synchronously before sendMessage so the create request carries the
  role.
- Show the current role's name in the window header badge and as the
  assistant's display name (transcript label + "… is typing…"), falling
  back to "AI agent" for a role-less chat. selectChat resets the picked
  role so it cannot leak into an unrelated existing chat.
- Add the "Take a look at the current document" i18n key (en-US, ru-RU).

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

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

@@ -669,6 +669,7 @@
   "AI Answer": "Ответ ИИ",
   "Ask AI": "Спросить ИИ",
   "AI agent": "AI-агент",
+  "Take a look at the current document": "Посмотри текущий документ",
   "AI agent is typing…": "AI-агент печатает…",
   "{{name}} is typing…": "{{name}} печатает…",
   "Agent role": "Роль агента",