Rewrite with modern stack

Major refactor from simple HTML/JS app to modern full-stack TypeScript application:

## Architecture Changes
- Migrated to monorepo structure with workspaces (backend, frontend, shared)
- Backend: Node.js + Express + TypeScript + Socket.IO
- Frontend: Next.js 15.5 + React 19 + TypeScript + Tailwind CSS
- Shared: Common types and utilities across packages

## Key Features Implemented
- Real-time WebSocket collaboration via Socket.IO
- Virtual canvas with chunked loading for performance
- Modern UI with dark mode and responsive design
- Mock database system for easy development (Redis/PostgreSQL compatible)
- Comprehensive error handling and rate limiting
- User presence and cursor tracking
- Infinite canvas support with zoom/pan controls

## Performance Optimizations
- Canvas virtualization - only renders visible viewport
- Chunked pixel data loading (64x64 pixel chunks)
- Optimized WebSocket protocol
- Memory-efficient state management with Zustand

## Development Experience
- Full TypeScript support across all packages
- Hot reload for both frontend and backend
- Docker support for production deployment
- Comprehensive linting and formatting
- Automated development server startup

## Fixed Issues
- Corrected start script paths
- Updated environment configuration
- Fixed ESLint configuration issues
- Ensured all dependencies are properly installed
- Verified build process works correctly

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/config/env.ts

@@ -0,0 +1,43 @@
+import dotenv from 'dotenv';
+
+dotenv.config();
+
+export const config = {
+  port: parseInt(process.env.PORT || '3001'),
+  host: process.env.HOST || 'localhost',
+  nodeEnv: process.env.NODE_ENV || 'development',
+  jwtSecret: process.env.JWT_SECRET || 'your-super-secret-jwt-key-change-in-production',
+  corsOrigin: process.env.CORS_ORIGIN ? process.env.CORS_ORIGIN.split(',') : ['http://localhost:3000'],
+  
+  // Rate limiting
+  rateLimits: {
+    pixelsPerMinute: parseInt(process.env.RATE_LIMIT_PIXELS_PER_MINUTE || '60'),
+    pixelsPerHour: parseInt(process.env.RATE_LIMIT_PIXELS_PER_HOUR || '1000'),
+    cursorUpdatesPerSecond: parseInt(process.env.RATE_LIMIT_CURSOR_PER_SECOND || '10'),
+  },
+
+  // Canvas settings
+  canvas: {
+    maxSize: parseInt(process.env.MAX_CANVAS_SIZE || '10000'),
+    defaultSize: parseInt(process.env.DEFAULT_CANVAS_SIZE || '1000'),
+    chunkSize: parseInt(process.env.CHUNK_SIZE || '64'),
+  },
+
+  // Redis settings
+  redis: {
+    url: process.env.REDIS_URL || 'redis://localhost:6379',
+    keyPrefix: process.env.REDIS_KEY_PREFIX || 'gaplace:',
+  },
+
+  // Logging
+  logLevel: process.env.LOG_LEVEL || 'info',
+} as const;
+
+export function validateConfig(): void {
+  const required = ['JWT_SECRET'];
+  const missing = required.filter(key => !process.env[key]);
+  
+  if (missing.length > 0 && config.nodeEnv === 'production') {
+    throw new Error(`Missing required environment variables: ${missing.join(', ')}`);
+  }
+}