(ui) Highlight current navigation section

Mark the active primary or system navigation link with aria-current and a
subtle visual state so staff users can see where they are in the control
panel without making Admin a primary app route.

Refs #37

src/pobsync_backend/tests/test_views.py

@@ -55,6 +55,15 @@ class ViewTests(TestCase):
         self.assertContains(response, reverse("changelog"))
         self.assertContains(response, "/api/status/")
         self.assertContains(response, reverse("admin:index"))
+        self.assertContains(response, '<a href="/" aria-current="page">Dashboard</a>', html=False)
+
+    def test_base_navigation_marks_current_secondary_page(self) -> None:
+        self.client.force_login(self.staff_user)
+
+        response = self.client.get(reverse("self_check"))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, f'<a href="{reverse("self_check")}" aria-current="page">Self Check</a>', html=False)
 
     def test_changelog_requires_staff_login(self) -> None:
         response = self.client.get(reverse("changelog"))