Add settings UI page (#171)

* Add theme colors from Figma

* Rough-in of AppHeader

* Add styled ActionButton

* Add react-router and placeholder Settings page

* Add ability to set persistent defaultDir

* Add react-hot-toast for save success message

* Add defaultProjectName setting

* Handle case of stale empty defaultDir in storage

* Wrap app in BrowserRouter

* Wrap test App in BrowserRouter

* Don't need BrowserRouter outside of testing
because we use RouterProvider

src/components/ErrorPage.tsx

@@ -0,0 +1,8 @@
+export const ErrorPage = () => {
+  return (
+    <div className="flex flex-col items-center justify-center h-screen">
+      <h1 className="text-4xl font-bold">404</h1>
+      <p className="text-2xl font-bold">Page not found</p>
+    </div>
+  )
+}