## Summary
- Adds a `previewIdleTimeoutPolicy` enum setting (`"default"` |
`"never"`) that controls the 10-minute idle GC for app previews.
- When set to `"never"`, the idle GC is skipped so previews stay warm
across app switches (more memory, faster preview loads).
- Exposes a new toggle in Workflow Settings: **Keep app previews running
forever** (disabled by default).

## Test plan
- [ ] Leave the toggle off: open an app preview, switch to another app,
wait 10+ minutes — original preview should be stopped as before.
- [ ] Turn the toggle on: do the same — the original preview should keep
running after the idle window.
- [ ] Toggling off should restore GC on the next minute-interval.
- [x] All existing unit tests pass (snapshots updated).
- [x] Lint + typecheck pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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---------
Co-authored-by: Will Chen <7344640+wwwillchen@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>