fix: move glassmorphism to shell level, sidebar becomes transparent

Shell now has the glass treatment so sidebar and surrounding area
are one seamless surface. No more visible line between sidebar
and content panel. Desktop sidebar is transparent over the shell.
Mobile sidebar keeps its own glass since it overlays independently.

components/dashboard/DashboardShell.tsx

@@ -11,7 +11,7 @@ const Sidebar = dynamic(() => import('./Sidebar'), {
   // so page content never occupies the sidebar zone
   loading: () => (
     <div
-      className="hidden md:block shrink-0 bg-neutral-950 overflow-hidden relative"
+      className="hidden md:block shrink-0 bg-transparent overflow-hidden relative"
       style={{ width: 64 }}
     >
       <div className="absolute inset-0 bg-gradient-to-r from-transparent via-neutral-800/10 to-transparent animate-shimmer" />
@@ -31,7 +31,7 @@ export default function DashboardShell({
   const openMobile = useCallback(() => setMobileOpen(true), [])
 
   return (
-    <div className="flex h-screen overflow-hidden bg-neutral-950">
+    <div className="flex h-screen overflow-hidden bg-neutral-900/65 backdrop-blur-3xl backdrop-saturate-150 supports-[backdrop-filter]:bg-neutral-900/60">
       <Sidebar
         siteId={siteId}
         mobileOpen={mobileOpen}