UI/Qt: Preserve macOS web modifier semantics

Qt swaps Command into ControlModifier and Control into MetaModifier on
macOS so shortcut definitions can stay portable. Convert those modifiers
back to Web-facing semantics before sending input events to LibWeb.

This lets Command-click behave as platform-control click instead of
being mistaken for Control-click, while physical Control-click still
opens the context menu on macOS.