Add a ChatGPT-style conversation list beside the embedded TUI on the
dashboard Chat tab so users can swap sessions without leaving the page.
- New ChatSessionList component: lists recent sessions for the active
profile (title/preview, last-active, message count, source), a New chat
button, and a refresh control. Best-effort like ChatSidebar.
- Selecting a row drives /chat?resume=<id>, which ChatPage already treats
as part of the PTY identity, so the terminal respawns resuming that
conversation. Active row is highlighted; New chat clears resume.
- Wired into ChatPage as a dedicated right-side column (desktop) and into
the existing slide-over panel above model/tools (narrow screens).
- i18n: new sessions.newChat key across all locales.
- Read-only switcher by design — delete/rename/export stay on Sessions.
Docs: web-dashboard.md Chat section documents the switcher.
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