Double-in rule support

[BySplashGm]

Description

The doubleIn field exists on the Game model but is not enforced in GameService / GameNotifier. When enabled, players must hit a double before their score starts counting.

Acceptance criteria

• GameService.recordThrow() checks game.doubleIn; ignores throws until the player lands a double
• GameState tracks a hasStarted flag per player
• ActiveGameScreen shows a "Need a double to start" hint for players who haven't started yet
• Game setup screen exposes a Double-in toggle (next to the existing Double-out rule)
• Unit tests cover the double-in entry condition