Lobby settings and error handling

Added

• Host and lobby settings carry contract-bound validation and a structured error envelope.
• Bot timing and reconnect grace windows come from environment settings.
• Backend error codes translate into readable client messages, and VERSION supports pre-release tags.

Changed

• CSS variables and pixel-corner styling are centralized as the Tailwind rollout begins.

Fixed

• Host settings clamp to contract bounds, rank colours apply to tied players, and duplicate toasts are gone.