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]>

.gitignore

@@ -1,5 +1,6 @@
 app/node_modules/
 app/dist/
+dist/
 *.log
 .DS_Store
 Thumbs.db