Add shared-link multiplayer mode

[Copilot]

Summary

Adds a shared-link multiplayer mode: a player can start a game, share a room link, and friends who open it join the same room and play the same secret word together, with guesses syncing live for everyone.

Changes

• server/: small Node + ws WebSocket server that owns room state (secret word, shared guess list, connected players) in-memory. Rooms are created lazily on first join and cleaned up 5 minutes after everyone leaves.
• src/multiplayer/: socketClient.js (WebSocket connection helper) and useRoomConnection.js (React hook exposing room state + guess/reset actions).
• src/components/Multiplayer/: Lobby (create room button), JoinPrompt (enter display name), MultiplayerGame (shared board + player list + share link, reuses existing GuessForm/GuessResults/ResultsBanner), PlayerList.
• App.js: reads a ?room= query param to switch between the existing single-player view and the new multiplayer view (no routing library needed, avoids SPA-fallback concerns with the Parcel dev/build setup).
• README: instructions for running the multiplayer server alongside the app.

Design notes

• Scope was intentionally kept aligned with the existing app: GuessCells already computes results client-side from an answer prop that's already visible in React state/console (console.info({ answer }) in the original Game.js), so the server is the source of truth for keeping room state in sync across clients rather than a security boundary / anti-cheat layer.
• Single-player mode at / is untouched; multiplayer lives behind ?room=<id>.

Testing

• npx eslint src/ — no errors.
• npx parcel build public/index.html — builds successfully.
• Manual server test: two WebSocket clients joining the same room see the same answer/room id, and guesses broadcast correctly.
• End-to-end Playwright test with two browser contexts: create room -> copy share link -> second browser joins via link -> both see the same player list -> guess submitted by one player appears live for both. All steps passed.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com