Lets players leave the tutorial at any time via a top-left link back to the
game session. Once the tutorial finishes and the input field is disabled,
the button relocates into the input row so it's the obvious next action.
Adds a black-terminal tutorial page at /game/pregame/{session} that walks
players through help, pwd, cd, ls, cat, rm and sudo before the real game
starts. Links to it are shown on both the lobby (waiting for players) and
ready-up screens so players can practice, repeatedly, before starting.
Frank