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/Shell/MenuSerializer.php

@@ -132,6 +132,10 @@ class MenuSerializer
       return "";
     }
 
+    if ($slug === "plugins.php") {
+      return admin_url("plugins.php?page=aurora-admin-plugins");
+    }
+
     // Already an absolute URL.
     if (preg_match('#^https?://#', $slug)) {
       return $slug;