Add frontend JS and improve post styling

Frontend is built with Bun. It uses Stimulus to progressively enhance
the server-built HTML. Currently, it only replaces UTC timestamps from
the server with the time in the browser's timezone.

frontend/globals.d.ts

@@ -0,0 +1,5 @@
+import { Application } from "@hotwired/stimulus";
+
+declare global {
+  var Stimulus: Application;
+}