feat: Completed Milestone 4 - Launchpad Site Builder

- Implemented Full Site Builder Suite:
- Sites Manager (Multi-site deployment management)
- Site Dashboard (Tabs for Pages, Nav, Theme)
- Page Editor (Visual Block Editor for Hero, Content, Features)
- Navigation Manager (Menu editor)
- Theme Settings (Global styles)
- Backend Schema (sites, pages, navigation, globals) setup script
- Integrated all components into /admin/sites pages

frontend/src/pages/admin/sites/editor/[pageId].astro

@@ -0,0 +1,10 @@
+---
+import Layout from '@/layouts/AdminLayout.astro';
+import PageEditor from '@/components/admin/sites/PageEditor';
+
+const { pageId } = Astro.params;
+---
+
+<Layout title="Page Editor | Spark Launchpad" hideSidebar={true}>
+    <PageEditor client:only="react" pageId={pageId!} onBack={() => history.back()} />
+</Layout>