fix(datepicker): reset a stale overlay before reattaching it

Navigating back while a mobile picker was open reattached its overlay still
active, leaving the page covered by an overlay nothing could close. Also skip
the reattach when the global container doesn't exist yet.

frontend/src/js/datepicker.js

@@ -117,8 +117,10 @@ let mobileOverlay = null;
 function createAirDatepicker(element, opts) {
     const datepicker = new AirDatepicker(element, opts);
     mobileOverlay ??= document.querySelector('.air-datepicker-overlay');
-    if (mobileOverlay && !mobileOverlay.isConnected) {
-        document.getElementById(AirDatepicker.defaultGlobalContainerId).appendChild(mobileOverlay);
+    const container = document.getElementById(AirDatepicker.defaultGlobalContainerId);
+    if (mobileOverlay && !mobileOverlay.isConnected && container) {
+        mobileOverlay.classList.remove('-active-'); // its picker was swapped out while open
+        container.appendChild(mobileOverlay);
     }
     return datepicker;
 }