feat: Session Preview / Summary in Task List

## Summary

Show a short preview snippet (last message) for each task in the LeftNav task list, so users can quickly see what each session is about without opening it.

## Gateway API

The Gateway already exposes `sessions.preview`:

```
sessions.preview → { sessionId } → { snippet: string }
```

Call it for each visible task and display the returned snippet under the task title in LeftNav.

## Scope

- Call `sessions.preview` for visible tasks in the LeftNav
- Display the last message snippet below the task title
- Handle loading / empty states gracefully
- No local caching needed — Gateway is source of truth

## Acceptance Criteria

- [ ] Each task in LeftNav shows a one-line preview of the last message
- [ ] Preview updates when a new message arrives in the session
- [ ] Empty sessions show a sensible placeholder
- [ ] No performance regression when many tasks are visible (batch or lazy-load as needed)

Start at the LeftNav task list and inspect how visible tasks and session message updates are currently loaded. Use the Gateway’s sessions.preview({ sessionId }) response, then verify one-line previews, loading and empty states, updates after new messages, and acceptable behavior with many visible tasks.

Written by the indexing model from the issue text.