feat: web UI redesign with PWA support, voice page, and mobile layout

- Redesign dash/history/notifications/trace pages with unified CSS framework
  (ui.css): cards, badges, scrollable tables, responsive grid, PWA shell.
- Add /voice page with chat-bubble transcript, listening indicator, and
  push-to-talk button for voice interaction.
- Add mavweb.js: client-side JS for live transcript polling, auto-scroll,
  push-to-talk, and service-worker registration.
- Update PWA manifest, icon, and service-worker for installable web app.
- Add WebAuthn credential management (register/list/delete) with modal UI.
- Update main.go: /voice route, WebAuthn handlers, new page navigation.
- Update handlers_test.go for new voice and WebAuthn routes.

cmd/mavweb/static/service-worker.js

@@ -1,6 +1,6 @@
 const CACHE = "maven-v2";
 self.addEventListener("install", e => {
-  e.waitUntil(caches.open(CACHE).then(c => c.addAll(["/", "/manifest.json", "/dash"])));
+  e.waitUntil(caches.open(CACHE).then(c => c.addAll(["/", "/ui.css", "/mavweb.js", "/manifest.json", "/dash"])));
   self.skipWaiting();
 });
 self.addEventListener("activate", e => {