feat(renderer): App.tsx renders AppShell

src/renderer/App.tsx

@@ -1,26 +1,7 @@
-import { ThemeToggle } from './components/theme-toggle';
+import { AppShell } from './components/layout/AppShell';
-import { Button } from './components/ui/button';
 
 function App() {
-  return (
+  return <AppShell />;
-    <div className="flex h-screen flex-col bg-background text-foreground">
-      <header className="flex h-14 items-center justify-between border-b border-border px-4">
-        <div className="flex items-center gap-3">
-          <div className="h-7 w-7 rounded-md bg-brand" aria-label="MarkdownConverter logo" />
-          <h1 className="text-sm font-semibold">MarkdownConverter</h1>
-        </div>
-        <div className="flex items-center gap-2">
-          <Button variant="ghost" size="sm">File</Button>
-          <Button variant="ghost" size="sm">Edit</Button>
-          <Button variant="ghost" size="sm">View</Button>
-          <ThemeToggle />
-        </div>
-      </header>
-      <main className="flex flex-1 items-center justify-center text-muted-foreground">
-        <p>Shell skeleton — implementation phases in progress.</p>
-      </main>
-    </div>
-  );
 }
 
 export default App;