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/style.css

@@ -765,7 +765,8 @@ h1 {
   font-size: 1rem;
 }
 
-button[type="submit"] {
+button[type="submit"],
+.btn-primary {
   padding: 0.6rem 1rem;
   border: none;
   border-radius: 8px;
@@ -775,7 +776,14 @@ button[type="submit"] {
   cursor: pointer;
 }
 
-button[type="submit"]:disabled {
+.btn-primary {
+  display: block;
+  width: 100%;
+  text-align: center;
+}
+
+button[type="submit"]:disabled,
+.btn-primary:disabled {
   opacity: 0.6;
   cursor: progress;
 }