Show user-friendly error when passkey authentication fails

Add error target to login page so WebAuthn errors are visible instead
of only appearing in the console. Use a helpful fallback message that
suggests a browser extension may be interfering.

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

app/views/sessions/new.html.erb

@@ -38,6 +38,7 @@
         </svg>
         Continue with Passkey
       </button>
+      <div data-webauthn-target="error" class="mt-2 text-sm text-red-600" style="display: none;"></div>
     </div>
 
     <!-- Password section - shown by default, hidden if WebAuthn is required -->