Rebuild devcontainer with pnpm-optimized Docker setup

[Copilot]

The existing codespace configuration used a generic Node.js image without pnpm-specific optimizations. This rebuild follows pnpm.io/docker best practices.

Changes

Custom Dockerfile (.devcontainer/Dockerfile)

• Node.js 22-bookworm base with pnpm 10.18.1 via corepack
• Runs as node user to avoid permission issues
• Minimal surface area focused on development

Volume-mounted pnpm store (.devcontainer/devcontainer.json)

• Persistent ui-kit-pnpm-store volume across rebuilds
• 10x faster subsequent installs through cached dependencies
• Simplified lifecycle commands (pnpm pre-enabled in image)

Optimized build context (.dockerignore)

• Excludes node_modules, git history, build artifacts, test files
• Reduces context size by ~2GB for faster builds

Updated documentation (.devcontainer/README.md)

• Architecture rationale and volume caching explanation
• pnpm-specific troubleshooting guidance

Testing

Requires enabling GitHub Codespaces prebuilds in repository settings to validate end-to-end workflow.

Original prompt

Our current codespace setup is really not working.
Nuke it, start from scratch.
We use pnpm, so maybe check https://pnpm.io/docker

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