Rename shared package to api-contracts for client/server DTO contracts

[Copilot]

This updates the client/server contract package name to reflect its actual scope: API DTO/type sharing only. The old shared/chess-shared naming has been replaced with api-contracts consistently across package resolution and imports.

• Package identity + location

  • Renamed workspace folder shared/ → api-contracts/.
  • Updated package name in api-contracts/package.json from chess-shared to api-contracts.

• Consumer import updates

  • Replaced all chess-shared imports with api-contracts in client and server code paths using lobby DTOs.

• Local linking workflow

  • Updated scripts/link-packages.sh to link/unlink api-contracts and resolve REPO_ROOT/api-contracts instead of shared.

// before
import { LobbyUpdateDto } from "chess-shared";

// after
import { LobbyUpdateDto } from "api-contracts";