feat: app shell avec sidebar, header et pages placeholder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/(dashboard)/layout.tsx

@@ -0,0 +1,10 @@
+import { Sidebar } from '@/components/layout/Sidebar'
+
+export default function DashboardLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <div className="flex min-h-screen bg-gray-50">
+      <Sidebar />
+      <main className="flex-1 flex flex-col overflow-hidden">{children}</main>
+    </div>
+  )
+}