chore(gateway-ui): bump service worker cache to v3

Force cache refresh after Tailwind UI redesign so clients pick up the
new styles and markup.

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

src/gateway/ui/sw.js

@@ -1,4 +1,4 @@
-const CACHE_NAME = 'flynn-webchat-v2';
+const CACHE_NAME = 'flynn-webchat-v3';
 const token = new URL(self.location.href).searchParams.get('token');
 const withToken = (path) => {
   if (!token) {