Style the passkey login button to match the email sign-in button

Introduce a .btn-primary class carrying the same look as
button[type="submit"] (accent fill, full width, centered), and apply
it to the passkey button on LoginView -- previously unstyled since it
is type="button", not type="submit".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/views/LoginView.vue

@@ -56,7 +56,7 @@ async function onPasskeyLogin() {
     <h1>Log in</h1>
 
     <template v-if="passkeySupported">
-      <button type="button" :disabled="passkeySubmitting" @click="onPasskeyLogin">
+      <button type="button" class="btn-primary" :disabled="passkeySubmitting" @click="onPasskeyLogin">
         {{ passkeySubmitting ? 'Waiting for your passkey…' : 'Log in with a passkey' }}
       </button>
       <p v-if="passkeyError" class="form-error">{{ passkeyError }}</p>