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/pages/dashboard/components/AuroraCard.vue

@@ -30,7 +30,11 @@ defineProps({
   display: flex;
   flex-direction: column;
   height: 100%;
-  background: var(--aurora-card-bg);
+  /* Deeper than the generic --aurora-card-bg — the Dashboard is now
+     wrapped in its own frame (windowChrome.js's .aurora-dashboard-frame,
+     using --aurora-frame-bg), and that's close in tone to --aurora-card-bg
+     for most presets, so cards would nearly disappear into it otherwise. */
+  background: var(--aurora-card-bg-deep, var(--aurora-card-bg));
   border: 1px solid var(--aurora-card-border);
   border-radius: 20px;
   padding: 20px;