Add optional StoreKit support tip (features stay free), redesign the
landing page with zh/en + light/dark and product shots, and ship
iPhone/iPad/Mac App Store screenshot sets.
Remove KeyboardL10n hard-coded fallbacks in favor of ExtL10n and extension
Localizable.strings. Add Flow session expiry hints, Darwin cross-process
notifications, session monitor on the keyboard, and app icon on GitHub Pages.
Rocky