Cleanup: remove unused stop_event in tray manager

## Problem
`lb_ui/services/tray.py` has an unused `stop_event` variable.

## Evidence
- `arch_report/by_target/lb_ui/vulture.txt:1`

## Plan
1. Remove the unused variable or wire it into lifecycle handling.
2. Run UI unit tests.

## Validation
- `uv run pytest tests/unit/lb_ui/test_*tray*.py`
- `./scripts/arch_audit.sh lb_ui`
