Closes #1971













<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Virtualized the chat message list with react-virtuoso for smoother
scrolling and lower memory use in long chats, with non-virtualized
rendering in E2E builds for stable tests. Memoized ChatMessage to reduce
unnecessary re-renders.

- **New Features**
- Switched MessagesList to Virtuoso with itemContent, overscan, smooth
follow, and initial focus on the latest message.
- Moved context limit and Undo/Retry controls into a footer; preserved
empty/setup states.

- **Dependencies**
  - Added react-virtuoso ^4.17.0.

<sup>Written for commit f82a976c70bebf40d5c7af7de514e0afde604c64.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->













<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Virtualizes the chat message list with react-virtuoso, adds a
non-virtualized path for E2E via E2E_TEST_BUILD, and refactors
footer/actions with memoized ChatMessage.
> 
> - **Frontend (chat)**:
> - Switch `src/components/chat/MessagesList.tsx` to `react-virtuoso`
with `itemContent`, `components.Footer`, overscan, and smooth follow;
extract `FooterComponent` for context limit + Undo/Retry controls.
> - Add E2E test mode using `envVars.E2E_TEST_BUILD` to render
non-virtualized list; improve empty/setup handling; memoize
`ChatMessage`.
> - **Electron/IPC**:
> - Include `E2E_TEST_BUILD` in `get-env-vars` so renderer can access
test-mode flag.
> - **Dependencies**:
>   - Add `react-virtuoso`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
1c71fe7555a6a32ff29ef1034b889822527484ba. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->