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/manifest.json

@@ -3,8 +3,8 @@
   "short_name": "Maven",
   "start_url": "/",
   "display": "standalone",
-  "background_color": "#111",
-  "theme_color": "#00aaff",
+  "background_color": "#14110D",
+  "theme_color": "#A78BFA",
   "icons": [
     {
       "src": "/icon.svg",