Add remember me checkbox, center and narrow sign-in form

- Add "Remember me for 30 days" checkbox (30-day vs 24-hour session expiry)
- Center heading and constrain form width to max-w-md
- Preserve remember_me preference through TOTP and WebAuthn auth flows

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/javascript/controllers/webauthn_controller.js

@@ -180,7 +180,8 @@ export default class extends Controller {
           "X-CSRF-Token": this.getCSRFToken()
         },
         body: JSON.stringify({
-          email: this.getUserEmail()
+          email: this.getUserEmail(),
+          remember_me: this.getRememberMe()
         })
       });
 
@@ -295,6 +296,11 @@ export default class extends Controller {
     return emailInput ? emailInput.value.trim() : "";
   }
 
+  getRememberMe() {
+    const checkbox = document.querySelector('input[name="remember_me"][type="checkbox"]');
+    return checkbox ? checkbox.checked : false;
+  }
+
   isValidEmail(email) {
     return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
   }