fix: include attachment text in chat payload

[embire2]

Summary

• extract text and PDF attachments and include them in a fenced ATTACHMENTS block in the user message payload
• keep attachment text aligned with uploads and screenshots
• enforce size limits and truncate long attachment text
• add pdfjs-dist for client-side PDF text extraction

Testing

• pnpm run typecheck
• pnpm run lint
• e2e: Playwright smoke (drag/drop a text attachment, send message, assert /api/chat payload includes ATTACHMENTS, filename, and content)