feat: static header + collapsible sidebar navigation

Replace floating pill header with static variant for authenticated
views. Add collapsible sidebar with site picker, grouped navigation
(Analytics/Infrastructure), and mobile overlay drawer. Remove
horizontal SiteNav tab bar.

components/dashboard/DashboardShell.tsx

@@ -0,0 +1,23 @@
+'use client'
+
+import Sidebar from './Sidebar'
+import { useSidebar } from '@/lib/sidebar-context'
+
+export default function DashboardShell({
+  siteId,
+  children,
+}: {
+  siteId: string
+  children: React.ReactNode
+}) {
+  const { mobileOpen, closeMobile } = useSidebar()
+
+  return (
+    <div className="flex flex-1 min-h-0">
+      <Sidebar siteId={siteId} mobileOpen={mobileOpen} onMobileClose={closeMobile} />
+      <main className="flex-1 min-w-0 overflow-auto pb-8">
+        {children}
+      </main>
+    </div>
+  )
+}