Show the passage during the countdown, and fix navigation across screens

Countdown
- Five seconds instead of three, and shown over the passage on the race
  screen rather than in an empty lobby. New ServerMessage::RaceText carries
  the text (and its attribution) just before the countdown starts, so every
  client can put it on screen and let players read the opening words while
  the numbers run - what TypeRacer and 10FastFingers both do. Typing is
  locked until Start lifts the gate.
- Two subscriptions had to move with it. Building the race view tears the
  lobby down, and the lobby owned the only handlers for Countdown and Start,
  so the count stopped arriving exactly when it was needed and the gate would
  never have lifted.
- The TUI and Steam clients match ServerMessage exhaustively, so both gained
  an arm for the new variant; whole workspace checks clean.

Multiplayer attribution
- Race passages now carry who wrote them, so a race shows its source the way
  single player always has. Previously the server sent bare strings and both
  the race and its end screen showed nothing.

Navigation
- Every screen's full-width "Back" button becomes a close control at the top
  of the content. The wordmark and Escape already went back, so it was a
  third way to do the same thing and the least reachable of them.
- The multiplayer screen had a Back button on top of that; gone.

Fixes
- The end screen graph is drawn into a canvas, so it cannot inherit a theme
  change the way the rest of the page does. It now redraws on one - switching
  theme used to leave the chart in the old palette.
- .stats-empty was align-self:flex-start inside a centred column, so "Sign in
  to add friends" sat on a different axis from its own Sign In button.
- The store showed a sign-in prompt instead of its catalogue, so nobody could
  see what signing in would get them. The catalogue now renders signed out,
  with prices, behind a sign-in note.
- The room code field is sized for the five characters a code actually is,
  and Join carries the same weight as the field it belongs to.

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