fix: add dark theme support for "open in new tab" FAB button

The backdrop-blur background on the iframe "open in new tab" floating
button was hardcoded to bg-white/80, making it look broken in dark
theme. Added dark:bg-dark-800/80 variant for both PurchaseSubscription
and CustomPage views.

frontend/src/views/user/CustomPageView.vue

@@ -160,7 +160,7 @@ onUnmounted(() => {
 
 .custom-open-fab {
   @apply absolute right-3 top-3 z-10;
-  @apply shadow-sm backdrop-blur supports-[backdrop-filter]:bg-white/80;
+  @apply shadow-sm backdrop-blur supports-[backdrop-filter]:bg-white/80 dark:supports-[backdrop-filter]:bg-dark-800/80;
 }
 
 .custom-embed-frame {

frontend/src/views/user/PurchaseSubscriptionView.vue

@@ -143,7 +143,7 @@ onUnmounted(() => {
 
 .purchase-open-fab {
   @apply absolute right-3 top-3 z-10;
-  @apply shadow-sm backdrop-blur supports-[backdrop-filter]:bg-white/80;
+  @apply shadow-sm backdrop-blur supports-[backdrop-filter]:bg-white/80 dark:supports-[backdrop-filter]:bg-dark-800/80;
 }
 
 .purchase-embed-frame {