xterm.js sends plain \r for Enter regardless of modifiers. Intercept
modified Enter in the custom key handler so it routes through CSI u
encoding (Kitty keyboard protocol), which Claude Code uses for
Shift+Enter newlines.
Only intercept keys when mobile virtual modifier buttons are active.
Previously all special keys (Enter, Ctrl+C, arrows) were intercepted
and manually re-encoded, bypassing xterm's built-in scrollOnUserInput.
