feat: content header with collapse toggle + realtime indicator

- New SidebarProvider context for shared collapse state
- ContentHeader visible on desktop: collapse icon left, "Live" right
- Collapse button removed from sidebar bottom (moved to header)
- Keyboard shortcut [ handled by context, not sidebar
- Realtime indicator polls every 5s, ticks every 1s for freshness

components/dashboard/DashboardShell.tsx

@@ -2,13 +2,12 @@
 
 import { useState, useCallback } from 'react'
 import dynamic from 'next/dynamic'
+import { SidebarProvider } from '@/lib/sidebar-context'
 import ContentHeader from './ContentHeader'
 
 // Load sidebar only on the client — prevents SSR flash
 const Sidebar = dynamic(() => import('./Sidebar'), {
   ssr: false,
-  // Placeholder reserves the sidebar's space in the server HTML
-  // so page content never occupies the sidebar zone
   loading: () => (
     <div
       className="hidden md:block shrink-0 bg-transparent overflow-hidden relative"
@@ -31,20 +30,21 @@ export default function DashboardShell({
   const openMobile = useCallback(() => setMobileOpen(true), [])
 
   return (
-    <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}
-        onMobileClose={closeMobile}
-        onMobileOpen={openMobile}
-      />
-      {/* Content panel — rounded corners, inset from edges. The left border doubles as the sidebar's right edge. */}
-      <div className="flex-1 flex flex-col min-w-0 mt-2 mr-2 mb-2 rounded-2xl bg-neutral-950 border border-neutral-800/60 isolate overflow-clip">
-        <ContentHeader onMobileMenuOpen={openMobile} />
-        <main className="flex-1 overflow-y-auto pt-4">
-          {children}
-        </main>
+    <SidebarProvider>
+      <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}
+          onMobileClose={closeMobile}
+          onMobileOpen={openMobile}
+        />
+        <div className="flex-1 flex flex-col min-w-0 mt-2 mr-2 mb-2 rounded-2xl bg-neutral-950 border border-neutral-800/60 isolate overflow-clip">
+          <ContentHeader siteId={siteId} onMobileMenuOpen={openMobile} />
+          <main className="flex-1 overflow-y-auto pt-4">
+            {children}
+          </main>
+        </div>
       </div>
-    </div>
+    </SidebarProvider>
   )
 }