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]>

admin/src/Shell/MenuSerializer.php

@@ -1,6 +1,8 @@
 <?php
 namespace AuroraAdmin\Shell;
 
+use AuroraAdmin\Options\Settings;
+
 defined("ABSPATH") || exit();
 
 /**
@@ -142,11 +144,14 @@ class MenuSerializer
       return "";
     }
 
-    if ($slug === "plugins.php") {
+    // Both checks default to true so existing installs keep their current
+    // behavior (previously always on regardless of these settings) until
+    // explicitly turned off in Settings > Plugins / Settings > Comments.
+    if ($slug === "plugins.php" && Settings::get("modern_plugins", true)) {
       return admin_url("plugins.php?page=aurora-admin-plugins");
     }
 
-    if ($slug === "edit-comments.php") {
+    if ($slug === "edit-comments.php" && Settings::get("modern_comments", true)) {
       return admin_url("admin.php?page=aurora-admin-comments");
     }