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/entries/plugins.js

@@ -0,0 +1,10 @@
+import { createApp } from 'vue';
+import '@/shell/shell-frame.css';
+import Plugins from '@/pages/plugins/Plugins.vue';
+
+const mountEl = document.getElementById('aurora-admin-plugins-root');
+
+if (mountEl) {
+  const data = JSON.parse(mountEl.dataset.auroraAdmin || '{}');
+  createApp(Plugins, { serverData: data }).mount(mountEl);
+}