fix: hide QR code container via inline style, not CSS

If applied, this commit will keep the QR code hidden until the
authentication/registration JS explicitly reveals it, without the
image becoming permanently invisible afterwards.

Why is this change needed?
Prior to this change, the QR image was hidden with a static
`display: none` CSS rule on `.qr img`. SlideableComponent.show()
reveals the page by sliding the `.qr`/`#qr` container down and then
stripping the inline styles jQuery added during the animation, but it
never touches the image's own display value. Once the CSS rule hid the
image, nothing ever set it back to visible, so the QR code disappeared
for good after the first show/hide cycle.

How does it address the issue?
This change reverts the CSS change and instead hides the `.qr`/`#qr`
container itself with an inline `style="display: none;"`, matching the
existing pattern already used for `#open-in-app` in the same
templates. Inline styles are fully replaced during the slide animation
and removed afterwards, so there is no leftover CSS rule to re-hide
the element on subsequent cycles.

Author: kayjoosten

assets/scss/application.scss

@@ -138,7 +138,7 @@ div.middle {
 
 .qr {
   img {
-    display: none;
+    display: block;
     margin-left: auto;
     margin-right: auto;
     cursor: none;

templates/default/authentication.html.twig

@@ -52,7 +52,7 @@
         <br /><br />
     </div>
 
-    <div id="qr" class="content-container qr">
+    <div id="qr" class="content-container qr" style="display: none;">
         {{ 'login.qr.instruction' | trans }}
 
         <a href={{ authenticateUrl }}>

templates/default/registration.html.twig

@@ -51,7 +51,7 @@
             <a href="{{ path('app_identity_registration') }}">{{ 'enrol.retry' | trans }}</a>.
         </div>
     </div>
-    <div class="content-container qr">
+    <div class="content-container qr" style="display: none;">
         <a href="{{ metadataUrl }}">
             <img src="{{ url('app_identity_registration_qr', {'enrollmentKey': enrollmentKey}) }}">
         </a>