Add Playwright MCP server support to container

[vanpelt]

This PR adds the Playwright MCP (Model Context Protocol) server to the Catnip container, enabling Claude to interact with web browsers for automated testing and web scraping tasks.

Changes

• Added Playwright system dependencies (Chrome/Chromium libraries) to the Dockerfile
• Installed @playwright/mcp-server globally via npm alongside Claude Code and Gemini CLI
• Added Chromium browser installation using npx playwright install chromium
• Created MCP configuration file (.mcp.json) that configures the Playwright server with headless Chromium and no-sandbox mode
• Updated entrypoint script to copy the MCP configuration to the user's home directory

Implementation Notes

The Playwright MCP server is configured to run in headless mode with Chromium only, using the --no-sandbox flag for container compatibility. The configuration follows the official Claude Code MCP documentation structure and will be automatically available to Claude when the container starts.