fix: add image upload size limits for hackathon (#63)
- useUploadFile: add 5MB limit + image type validation for team
logo/banner uploads (was unrestricted)
- useUploadAvatar: tighten limit from 5MB to 2MB per issue request
- Submit page: switch from useUploadFile to useUploadSubmission
which has proper 50MB limit + broader file type support
Size limits:
- Profile avatar: 2MB (JPEG, PNG, WebP, GIF)
- Team logo/banner: 5MB (JPEG, PNG, WebP, GIF)
- Team files: 20MB (images + PDF)
- Submissions: 50MB (images, PDF, ZIP, video)
Closes #63
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
