Add real-time multiplayer support

[victorgalvez56]

Summary

Adds real-time multiplayer to the folio using Socket.IO and a Node.js server. Multiple visitors can see and interact with each other's cars in the same scene.

• Multiplayer engine: Socket.IO server relays car physics snapshots between players with client-side interpolation
• Lobby UI: 3-step pre-game flow (name → car color → car type) with localStorage persistence
• Remote car rendering: GLTF template cloning with per-player matcap body colors
• Physics sync: Client-authoritative snapshots at 20Hz, CANNON.js collision bodies for remote cars
• Bump system: Car-to-car collision impulses relayed via server with camera shake
• In-game chat: Toggle panel with Enter/Escape shortcuts and unread badge
• Minimap: Top-left canvas showing all players, heading arrow, and track outline
• Race track: Procedural circuit with CatmullRom curves, matcap barrier walls, asphalt surface, start/finish line
• World-space floor: Checkered grid shader using world coordinates
• Player labels: Floating name tags above each car

Architecture

client/         ← Original folio restructured as Vite project
server/         ← Node.js + Socket.IO + CANNON.js physics server
shared/         ← Constants (physics, network, spawn grid, colors)

The original single-player code is preserved in client/ with multiplayer additions layered on top.

Live demo

https://racing.victorgalvez.dev

Test plan

• npm install at root + cd server && npm install
• Start server: node server/src/index.js
• Start client: npm run dev --prefix client
• Open two browser tabs — both should see each other after lobby
• Verify car colors and car types sync between players
• Test bumper-car collisions
• Test chat messaging between tabs
• Verify minimap shows all players and track outline