Wire up remaining placeholder settings, implement menu search (v2.4.0)

- Custom dashboard, Modern plugins page, and Modern comments page
  toggles previously had no effect at all — their respective takeovers/
  redirects (DashboardTakeover, SubApps' Plugins/Comments handling,
  MenuSerializer's link rewrites) ran unconditionally regardless of the
  setting. All three now default to their previous always-on behavior
  but can actually be turned off.
- Menu search had zero implementation. Sidebar.vue now shows a real
  search box (gated on the setting) that filters both the WP-native menu
  and the Aurora nav section by title, auto-expanding matching sections.
- Removed "Modern posts table" and "Modern users page" toggles (and
  their now-empty Posts/Users settings categories) — both described a
  custom management page that was never built. A checkbox with nothing
  behind it is worse than no checkbox.

Verified all four live: dashboard/plugins/comments toggles correctly
flip between the Aurora and native experience and back; menu search
filters correctly and clears back to the full menu.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/src/pages/settings/settingsConfig.js

@@ -152,20 +152,6 @@ export const settingsCategories = [
       },
     ],
   },
-  {
-    id: 'posts',
-    title: 'Posts',
-    subtitle: 'Post management and display preferences',
-    icon: 'dashicons-admin-post',
-    fields: [
-      {
-        key: 'modern_posts',
-        type: 'toggle',
-        label: 'Modern posts table',
-        description: 'Use the Aurora posts management interface.',
-      },
-    ],
-  },
   {
     id: 'plugins',
     title: 'Plugins',
@@ -230,20 +216,6 @@ export const settingsCategories = [
       },
     ],
   },
-  {
-    id: 'users',
-    title: 'Users',
-    subtitle: 'User management and administration settings',
-    icon: 'dashicons-admin-users',
-    fields: [
-      {
-        key: 'modern_users',
-        type: 'toggle',
-        label: 'Modern users page',
-        description: 'Use the Aurora users management interface.',
-      },
-    ],
-  },
   {
     id: 'comments',
     title: 'Comments',