feat: import FIT activities into Garmin

Implement GarminUploader adapter around garminconnect library:
- Import activities using import_activity() not upload_activity()
- Treat duplicate activity responses as terminal success
- Raise GarminUploadBlocked for MFA to allow UI code collection
- Distinguish auth, transient, and other errors appropriately
- Helper functions for duplicate detection and activity ID extraction

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
   "python-multipart>=0.0.9,<1",
   "itsdangerous>=2.1,<3",
   "httpx>=0.27,<1",
+  "garminconnect>=0.2,<1",
 ]
 
 [project.optional-dependencies]