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

frontend/src/App.tsx

@@ -1,34 +1,15 @@
-import { useState } from 'react'
-import reactLogo from './assets/react.svg'
-import viteLogo from '/vite.svg'
-import './App.css'
+import { MainLayout } from "./components/layout/MainLayout"
 
 function App() {
-  const [count, setCount] = useState(0)
-
   return (
-    <>
-      <div>
-        <a href="https://vite.dev" target="_blank">
-          <img src={viteLogo} className="logo" alt="Vite logo" />
-        </a>
-        <a href="https://react.dev" target="_blank">
-          <img src={reactLogo} className="logo react" alt="React logo" />
-        </a>
-      </div>
-      <h1>Vite + React</h1>
-      <div className="card">
-        <button onClick={() => setCount((count) => count + 1)}>
-          count is {count}
-        </button>
-        <p>
-          Edit <code>src/App.tsx</code> and save to test HMR
+    <MainLayout>
+      <div className="flex flex-col gap-4">
+        <h1 className="text-2xl font-bold">Welcome to Election Analytics</h1>
+        <p className="text-muted-foreground">
+          Select a conversation from the sidebar or start a new one to begin your analysis.
         </p>
       </div>
-      <p className="read-the-docs">
-        Click on the Vite and React logos to learn more
-      </p>
-    </>
+    </MainLayout>
   )
 }