Autofocus PIN input on Web UI pin page

This makes it faster to input the PIN after clicking on the PIN
notification on the desktop, since you no longer need to click
the PIN input or focus on it by pressing Tab many times.

src_assets/common/assets/web/pin.html

@@ -11,7 +11,7 @@
     <h1 class="my-4">{{ $t('pin.pin_pairing') }}</h1>
     <form action="" class="form d-flex flex-column align-items-center" id="form">
       <div class="card flex-column d-flex p-4 mb-4">
-        <input type="text" pattern="\d*" placeholder="PIN" id="pin-input" class="form-control my-4" />
+        <input type="text" pattern="\d*" placeholder="PIN" autofocus id="pin-input" class="form-control my-4" />
         <button class="btn btn-primary">{{ $t('pin.send') }}</button>
       </div>
       <div class="alert alert-warning">