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/handlers_test.go

@@ -74,6 +74,10 @@ func (f *fakeCore) DisableTool(_ context.Context, name string) error {
 	return f.disableErr
 }
 
+func (f *fakeCore) DeleteTool(_ context.Context, name string) error {
+	return nil
+}
+
 func (f *fakeCore) ListTools(_ context.Context, status string) ([]ipc.Tool, error) {
 	if f.listErr != nil {
 		return nil, f.listErr