v3

- add tutorial (it turns out that lots of people may either not be familiar with the classic Minesweeper, or may not remember it if they played it a long time ago)

- long press/tag to uncover/flag on mobile can be inverted

- middle click/right click to pan/zoom on desktop can be inverted

- first click is always safe

- add a flag notification on mobile when flagging a tile, to make it more obvious that the long press was long enough