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/App/AuroraAdmin.php

@@ -8,6 +8,7 @@ use AuroraAdmin\Pages\SettingsPage;
 use AuroraAdmin\Rest\DashboardData;
 use AuroraAdmin\Analytics\Analytics;
 use AuroraAdmin\Rest\AnalyticsData;
+use AuroraAdmin\Rest\PluginsData;
 use AuroraAdmin\Posts\ScheduledDayFilter;
 use AuroraAdmin\WhiteLabel\WhiteLabel;
 use AuroraAdmin\Pages\SubApps;
@@ -39,6 +40,7 @@ class AuroraAdmin
     new DashboardData();
     new Analytics();
     new AnalyticsData();
+    new PluginsData();
     new ScheduledDayFilter();
     new WhiteLabel();
     new SubApps();