Merge master into feat/issue-639-webui-system-health-dashboard

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]>

tests/test_webui_system_health_dashboard.py

@@ -15,7 +15,8 @@ from starlette.testclient import TestClient
 
 from webui.app import create_app
 from webui.deployment_boundary import scan_text_for_client_secrets
-from webui.layout import NAV_ITEMS, render_page
+from webui.layout import render_page
+from webui.nav import iter_nav_items
 from webui.system_health import (
     STATUS_DEGRADED,
     STATUS_DOWN,
@@ -317,7 +318,10 @@ class TestNavAndRoute(unittest.TestCase):
         self.client = TestClient(create_app())
 
     def test_nav_contains_system_health(self):
-        self.assertIn((DASHBOARD_PATH, "System health"), NAV_ITEMS)
+        self.assertIn(
+            (DASHBOARD_PATH, "System health"),
+            [(item.href, item.label) for item in iter_nav_items()],
+        )
 
     def test_rendered_shell_links_dashboard(self):
         page = render_page(title="Home", body_html="<p>x</p>")