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/icon.svg

@@ -1,10 +1,13 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
   <defs>
     <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0%" stop-color="#1a1a2e"/>
-      <stop offset="100%" stop-color="#0d0d1a"/>
+      <stop offset="0%" stop-color="#221D17"/>
+      <stop offset="100%" stop-color="#14110D"/>
     </linearGradient>
   </defs>
   <rect width="512" height="512" rx="96" fill="url(#bg)"/>
-  <text x="256" y="340" font-family="system-ui,-apple-system,sans-serif" font-size="320" font-weight="700" fill="#00aaff" text-anchor="middle">M</text>
+  <text x="256" y="320" font-family="Geist,system-ui,-apple-system,sans-serif" font-size="280" font-weight="600" fill="#A78BFA" text-anchor="middle">M</text>
+  <circle cx="196" cy="392" r="14" fill="#A78BFA" opacity="0.55"/>
+  <circle cx="256" cy="392" r="14" fill="#A78BFA" opacity="0.8"/>
+  <circle cx="316" cy="392" r="14" fill="#A78BFA"/>
 </svg>