Add theme presets, gradient backgrounds, and macOS-style window chrome

Replaces the old light/dark toggle with 10 fixed color themes (plus
Black & Grey and White) that drive a coordinated gradient palette across
the whole admin UI, a Google Fonts picker, and a native-page dark-mode
stylesheet that now covers modals and plugin/theme card grids. Fixes
several background/gradient seams where the sidebar, toolbar, and content
frame were each painting their own independent copy of the page gradient
instead of sharing one continuous plane.

Also adds a macOS-style traffic-light title bar to every page (close,
minimize, maximize) and a persistent bottom taskbar so a minimized page
can be restored from anywhere, plus a Plugins management page.

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

admin/src/Pages/SubApps.php

@@ -24,6 +24,7 @@ class SubApps
     "aurora-admin-roles" => ["title" => "Role Editor", "entry" => "role-editor"],
     "aurora-admin-activity-log" => ["title" => "Activity Log", "entry" => "activity-log"],
     "aurora-admin-database" => ["title" => "Database Explorer", "entry" => "database-explorer"],
+    "aurora-admin-plugins" => ["title" => "Installed Plugins", "entry" => "plugins"],
   ];
 
   private static $hook_suffixes = [];
@@ -38,11 +39,12 @@ class SubApps
   public static function register_menus()
   {
     foreach (self::PAGES as $slug => $page) {
+      $parent = $slug === "aurora-admin-plugins" ? "plugins.php" : "aurora-admin";
       $hook = add_submenu_page(
-        "aurora-admin",
+        $parent,
         __($page["title"], "aurora-admin"),
         __($page["title"], "aurora-admin"),
-        "manage_options",
+        $slug === "aurora-admin-plugins" ? "activate_plugins" : "manage_options",
         $slug,
         [self::class, "render"]
       );
@@ -53,7 +55,7 @@ class SubApps
   public static function hide_from_native_submenu()
   {
     foreach (array_keys(self::PAGES) as $slug) {
-      remove_submenu_page("aurora-admin", $slug);
+      remove_submenu_page($slug === "aurora-admin-plugins" ? "plugins.php" : "aurora-admin", $slug);
     }
   }