Add Windows-friendly local web chat, webhook support, and file uploads

[erik-mingyang]

Summary

This PR adds a Windows-friendly way to use BioClaw without relying on WhatsApp.

What changed

• added a local web chat channel
• added a simple HTTP webhook entrypoint
• added file upload support for the local web chat
• stored uploaded files in the group workspace so the agent can access them directly
• improved the local chat UI with file cards, preview links, download buttons, and image previews
• added .env loading and a local-web example config
• improved Windows compatibility while preserving the existing macOS flow
• clarified Chinese documentation for Windows and local web usage

Why

This makes BioClaw easier to use for Windows users, especially users in China who may prefer a browser-based workflow over WhatsApp.

Validation

• npm run typecheck
• npx vitest run src/channels/whatsapp.test.ts src/formatting.test.ts src/db.test.ts

[Runchuan-BU]

This PR has merge conflicts — the channel and db code has been restructured since. Please rebase onto latest main. Note: Windows setup is now covered by scripts/setup.ps1 on main. Thank you!