feat: Set width and height of main view

This makes the view look nice in *both* the preferences panel and the
extension popup. This is achievable by setting a pre-defined width for
the popup and ensuring that the maximum width doesn't exceed the
preferences panel.

src/app/layout.tsx

@@ -8,7 +8,7 @@ const root = document.getElementById('root')
 if (root !== null) {
   ReactDOM.createRoot(root).render(
     <React.StrictMode>
-      <div className="bg-100 text-700 base16-katy">
+      <div className="h-96 w-[48rem] max-w-full bg-100 text-700 base16-katy">
         <RootPage />
       </div>
     </React.StrictMode>