Resolve the #638 shell landing against the #639 dashboard:
- webui/layout.py: drop the flat NAV_ITEMS tuple in favor of master's
grouped NAV_GROUPS nav-config module.
- webui/nav.py: register /system-health as a live item in the Health
group, satisfying issue #639 AC5 through the canonical nav source.
- docs/webui-local-dev.md: keep both additive sections (#638 shell and
#639 dashboard).
- tests/test_webui_system_health_dashboard.py: assert the nav entry via
iter_nav_items() instead of the removed NAV_ITEMS tuple.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>