## Summary
- Allow multiple Dyad apps to run simultaneously instead of stopping the
previous app when switching
- Add garbage collection that stops apps idle for 10+ minutes (unless
currently selected)
- Track `lastViewedAt` timestamp on running apps to determine
eligibility for GC
- Clean up all running apps when the Dyad application quits

## Test plan
1. Create two test apps in Dyad
2. Run the first app and verify it starts
3. Switch to the second app - verify the first app keeps running (check
processes)
4. Wait 10+ minutes without viewing the first app - verify it gets
garbage collected
5. Close Dyad completely - verify all running app processes are stopped

Closes #2819

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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---------
Co-authored-by: Will Chen <willchen90@gmail.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>