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]>

app/src/shell/components/Toolbar.vue

@@ -129,10 +129,11 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
 .toolbar {
   position: fixed;
   top: 0;
-  left: var(--aurora-sidebar-w);
+  left: calc(var(--aurora-sidebar-w) + var(--aurora-sidebar-gap) * 2);
   right: 0;
   height: var(--aurora-toolbar-h);
-  background: var(--aurora-app-bg);
+  /* No background — see Sidebar.vue; the fixed html/body gradient shows
+     through so the whole shell reads as one continuous background. */
   display: flex;
   align-items: center;
   justify-content: space-between;