Add support for uploading file attachments to Grist documents:
- GristClient.upload_attachment() method using multipart/form-data
- upload_attachment tool function with base64 decoding and MIME detection
- Tool registration in server.py
- Comprehensive unit tests (7 new tests)
Returns attachment ID for linking to records via update_records.
Bumps version to 1.3.0.
Move unit tests from tests/ to tests/unit/ for clearer separation
from integration tests. Update pyproject.toml testpaths and Makefile
test target to reflect the new structure.
