feat(ui): Implement core layout with Sidebar and MainLayout

frontend/src/components/layout/MainLayout.tsx

@@ -0,0 +1,35 @@
+import * as React from "react"
+import { SidebarProvider, Sidebar, SidebarContent, SidebarHeader, SidebarFooter, SidebarInset, SidebarTrigger } from "@/components/ui/sidebar"
+
+interface MainLayoutProps {
+  children: React.ReactNode
+}
+
+export function MainLayout({ children }: MainLayoutProps) {
+  return (
+    <SidebarProvider>
+      <div className="flex min-h-screen w-full">
+        <Sidebar role="complementary">
+          <SidebarHeader>
+            <div className="p-4 font-bold text-xl">EA Chatbot</div>
+          </SidebarHeader>
+          <SidebarContent>
+            {/* Navigation links or conversation history will go here */}
+          </SidebarContent>
+          <SidebarFooter>
+            <div className="p-4 text-xs text-muted-foreground">© 2026 Election Analytics</div>
+          </SidebarFooter>
+        </Sidebar>
+        <SidebarInset className="flex flex-col flex-1">
+          <header className="flex h-16 shrink-0 items-center gap-2 border-b px-4" role="navigation">
+            <SidebarTrigger />
+            <div className="font-semibold">Chat</div>
+          </header>
+          <main className="flex-1 overflow-auto p-6">
+            {children}
+          </main>
+        </SidebarInset>
+      </div>
+    </SidebarProvider>
+  )
+}