[jorge-huxley]

Summary

Bryton China-region accounts currently receive a "user not found" error because login uses the international backend. Add an independent Bryton region setting so activity sync and planned workout uploads use the backend where the account was created.

Closes #64.

Changes

• Add International/China selection in GUI settings and INTERVALSSYNC_BRYTON_REGION for the CLI, defaulting to International.
• Route activity login, FIT downloads, and workout uploads through the selected regional endpoints; retain explicit endpoint overrides.
• Document the setting and cover region validation, endpoint selection, CLI propagation, and settings persistence with regression tests.
• Include the release workflow checkout fix used to publish the tested candidate.

Verification

• All 291 offline tests passed on an isolated export of this branch using the existing Python environment (python -m pytest -q -p no:cacheprovider, with a writable temporary directory).
• git diff --check passed.
• v0.9.5-rc1, this branch, and the successful release workflow all point to a04bf1ad9d9d76cd1287a50758fba5878a323337.
• The reporter confirmed their China-region account works, including loading and uploading routes: China-region account #64 (comment)

Notes / follow-ups

The tested candidate is available at https://github.com/jorge-huxley/intervalssync/releases/tag/v0.9.5-rc1. A subsequent stable release is needed to deliver the fix through stable distribution channels.