feat: collect multipart attachment uploads

[cezary17]

Summary

• collect multipart uploads passed to file() and image() so resolveDisUI() returns Discord attachment metadata plus a files array for the matching multipart form parts
• rewrite multipart-backed file and image component urls to attachment://<filename> across file, section thumbnail, and gallery item rendering
• document the multipart upload flow and cover it with render tests for files, section accessories, and media galleries

Verification

• npm test
• npm run build