- update about page layout.
  use slide in page instead of a fixed position overlay.
  this works well on both desktop and mobile.
- minor updates on about page text content.
- update about page font-family.
- on main UI, put title button on the rhs.

This impl doesn't work well on mobile device.
It's suitable for small pop-up window, not good for long texts.

- bugfix: click expand/shrink stack/trail button should not follow href link.
- update UI a little. updated text for times button.
- add flags to enable unit test. disable test by default.

keyboard is useful on the desktop.
TAB and backspace is not supported yet.

added doc on how to add new operator button.

- implemented undo using snapshots.
  I kept recent 1000 snapshots in a bounded stack.
  note: snapshot must clone the underlying array instead of shadow copying.
- bugfix: when input invalid number like "2.3.", the error msg was not
  shown on #error-msg.
- use monospace font on #number-display

should be easy to use on portrait mode.
landscape mode may require additional work to be pleasant.

add function in fsm and add relevant buttons on UI.

show scrollbar on stack and trait when content overflows.

- feature: backspace should delete number in stack in idle mode
- add error handling for "division by 0"
- bugfix on isNumberKey(), it's more strict now
- show error msg in web UI

added unit test for it.