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]>

aurora-admin.php

@@ -3,7 +3,7 @@
  * Plugin Name: Aurora Admin
  * Plugin URI: https://aurora.co
  * Description: A modern, fast WordPress admin interface.
- * Version: 2.3.1
+ * Version: 2.4.0
  * Author: Aurora
  * Text Domain: aurora-admin
  * Domain Path: /languages/
@@ -14,7 +14,7 @@
 
 defined("ABSPATH") || exit();
 
-define("AURORA_ADMIN_VERSION", "2.3.1");
+define("AURORA_ADMIN_VERSION", "2.4.0");
 define("AURORA_ADMIN_PATH", plugin_dir_path(__FILE__));
 define("AURORA_ADMIN_URL", plugins_url("aurora-admin/"));