fix: mobile keyboard input in playground terminal (#2)

* fix: use onData for terminal input, fixing mobile keyboard support

Replace onKey with onData as the sole xterm.js input handler. onKey
relies on KeyboardEvent which mobile virtual keyboards don't fire;
onData receives terminal byte sequences from all input sources.

Also default to emacs mode on mobile (no Escape key on virtual
keyboards) and add tap-to-focus for opening the keyboard.

* fix: batch refreshLine into single write to prevent cursor flicker

Author: akesling