Add hidden command to send test error to Sentry

I haven't seen anything come in from prod yet, and it's hard to trigger one, maybe because the integration is React-specific? Or maybe it's... not working :p

I can send errors from dev! But just haven't _seeeen_ a prod error come in yet.

Maybe we're just squeaky clean tho :3

src/app/HomePage.js

@@ -36,6 +36,12 @@ function HomePage() {
 
   const [previewState, setPreviewState] = React.useState(null);
 
+  React.useEffect(() => {
+    if (window.location.href.includes("send-test-error-for-sentry")) {
+      throw new Error("Test error for Sentry");
+    }
+  });
+
   return (
     <Flex direction="column" align="center" textAlign="center" marginTop="8">
       <Box