fix(mobile): prevent white flash when opening inline webview
Set WKWebView transparent (backgroundColor=.clear, isOpaque=false) so
the app's background shows through during page load. Inject a
document-start script that keeps the HTML canvas transparent until
DOMContentLoaded, then detects the page's real background color and
smoothly fades it in over 0.25s. Works automatically in both dark and
light mode without needing an isDark parameter.