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.

lib/sidebar-context.tsx

@@ -0,0 +1,31 @@
+'use client'
+
+import { createContext, useCallback, useContext, useState } from 'react'
+
+interface SidebarContextValue {
+  mobileOpen: boolean
+  openMobile: () => void
+  closeMobile: () => void
+}
+
+const SidebarContext = createContext<SidebarContextValue>({
+  mobileOpen: false,
+  openMobile: () => {},
+  closeMobile: () => {},
+})
+
+export function SidebarProvider({ children }: { children: React.ReactNode }) {
+  const [mobileOpen, setMobileOpen] = useState(false)
+  const openMobile = useCallback(() => setMobileOpen(true), [])
+  const closeMobile = useCallback(() => setMobileOpen(false), [])
+
+  return (
+    <SidebarContext.Provider value={{ mobileOpen, openMobile, closeMobile }}>
+      {children}
+    </SidebarContext.Provider>
+  )
+}
+
+export function useSidebar() {
+  return useContext(SidebarContext)
+}