feat: add Dockerfile for web frontend with Caddy

frontend/packages/web/Dockerfile

@@ -0,0 +1,34 @@
+FROM node:22-alpine AS build
+WORKDIR /app
+RUN npm install -g pnpm
+
+# Copy package files for all workspaces
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
+COPY frontend/packages/web/package.json ./frontend/packages/web/
+COPY sdks/typescript/package.json ./sdks/typescript/
+
+# Install dependencies
+RUN pnpm install --filter @sandbox-agent/web...
+
+# Copy SDK source (with pre-generated types)
+COPY sdks/typescript ./sdks/typescript
+
+# Build SDK (just tsc, skip generate since types are pre-generated)
+RUN cd sdks/typescript && pnpm exec tsc -p tsconfig.json
+
+# Copy web source
+COPY frontend/packages/web ./frontend/packages/web
+
+# Build web
+RUN cd frontend/packages/web && pnpm exec vite build
+
+FROM caddy:alpine
+COPY --from=build /app/frontend/packages/web/dist /srv
+RUN cat > /etc/caddy/Caddyfile <<'EOF'
+:80 {
+	root * /srv
+	file_server
+	try_files {path} /index.html
+}
+EOF
+EXPOSE 80