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/Options/Settings.php

@@ -35,7 +35,12 @@ class Settings
         "schema" => [
           "type" => "object",
           "properties" => [
-            "dark_mode" => ["type" => "string"],
+            // Two producers write this key with different types: the
+            // toolbar's tri-state switcher sends 'light'/'dark'/'system'
+            // (string), the settings page's Dark Mode toggle sends a plain
+            // boolean. Leaving it untyped (falls through to
+            // additionalProperties) lets both save instead of one
+            // rejecting the other with a 400.
             "dashboard_card_order" => ["type" => "object"],
           ],
           "additionalProperties" => true,