fix(mobile-ux): guard ScrollToTopAsync JS interop to prevent prerendering runtime error

src/NexusReader.UI.Shared/Components/Organisms/ReaderCanvas.razor

@@ -432,8 +432,16 @@
     {
         try
         {
+            // Ensure the JS module is loaded and the component is fully rendered before invoking interop.
+            if (!_isJsInitialized)
+            {
+                await InitViewportDetectionAsync();
+            }
             var module = await EnsureViewportModuleAsync();
-            await module.InvokeVoidAsync("scrollToTop", ".reader-canvas");
+            if (module != null)
+            {
+                await module.InvokeVoidAsync("scrollToTop", ".reader-canvas");
+            }
         }
         catch (Exception ex)
         {