feat(ui): add PWA configuration with service worker and enhanced manifest

Adds service worker with network-first navigation (SPA fallback) and
cache-first static assets. Enhances web manifest with start_url, scope,
id, description, orientation, and maskable icon entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/main.tsx

@@ -15,6 +15,12 @@ import { TooltipProvider } from "@/components/ui/tooltip";
 import "@mdxeditor/editor/style.css";
 import "./index.css";
 
+if ("serviceWorker" in navigator) {
+  window.addEventListener("load", () => {
+    navigator.serviceWorker.register("/sw.js");
+  });
+}
+
 const queryClient = new QueryClient({
   defaultOptions: {
     queries: {