chore: add Docker build configuration

Add multi-stage Dockerfile and nginx configuration:

**Dockerfile**
- Stage 1: Build with Node.js 22 Alpine and pnpm 9
- Stage 2: Serve with nginx 1.27 Alpine
- Static export optimization
- Health check endpoint

**nginx.conf**
- Gzip compression for assets
- CORS headers for Canvas API
- Cache static assets (1 year)
- Don't cache HTML files
- Support for WebAssembly files
- Security headers
- Client max body size: 100M for large image uploads

Ready for containerized deployment.

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

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

Dockerfile

@@ -0,0 +1,41 @@
+# Multi-stage build for Next.js static export
+
+# Stage 1: Build the application
+FROM node:22-alpine AS builder
+
+# Install pnpm
+RUN corepack enable && corepack prepare pnpm@9.0.0 --activate
+
+# Set working directory
+WORKDIR /app
+
+# Copy package files
+COPY package.json pnpm-lock.yaml* ./
+
+# Install dependencies
+RUN pnpm install --frozen-lockfile
+
+# Copy application files
+COPY . .
+
+# Build the Next.js application (static export)
+RUN pnpm build
+
+# Stage 2: Production server with nginx
+FROM nginx:1.27-alpine AS runner
+
+# Copy custom nginx configuration
+COPY nginx.conf /etc/nginx/nginx.conf
+
+# Copy built static files from builder stage
+COPY --from=builder /app/out /usr/share/nginx/html
+
+# Expose port 80
+EXPOSE 80
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget --quiet --tries=1 --spider http://127.0.0.1/ || exit 1
+
+# Start nginx
+CMD ["nginx", "-g", "daemon off;"]