fix: restore WKWebView alpha to 1 when reparenting to article container

The GPU warmup set webView.alpha = 0 to keep it invisible while
attached to the window. makeUIView reparents it to the article
container but never restored alpha — articles rendered invisibly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/Features/Reader/Views/ArticleWebView.swift

@@ -82,6 +82,7 @@ struct ArticleWebView: UIViewRepresentable {
 
         let webView = ArticleRenderer.shared.webView
         webView.removeFromSuperview()
+        webView.alpha = 1
         webView.frame = container.bounds
         webView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
         container.addSubview(webView)