Automate weekly Garmin FIT device list sync with updater and tests

[cagnulein]

Motivation

• Keep the Garmin FIT product selector in src/settings.qml up to date by importing new products from Garmin's generated fit_profile.hpp and avoid manual updates.
• Provide a reproducible updater that preserves existing entries and excludes regional/variant noise to keep the selector manageable.

Description

• Add a scheduled GitHub Actions workflow /.github/workflows/update-garmin-fit-devices.yml that runs weekly (and on demand) to test and run the updater, check for changes, and open or update a PR when updates are found.
• Introduce scripts/update_garmin_fit_devices.py, a standalone updater that parses Garmin's fit_profile.hpp, determines eligible products, updates the ComboBox in src/settings.qml in-place, and emits a Markdown summary and GitHub Actions output flag.
• Implement robust parsing, validation, and safeguards in the updater including duplicate checks, QML structure validation, preservation of existing entries, deterministic ordering by product number, and configurable catalog source via --catalog.
• Add unit tests tests/test_update_garmin_fit_devices.py that cover catalog parsing, QML location and updates, idempotence, exclusion rules, CLI behavior, and edge cases.

Testing

• The workflow runs python3 -m unittest tests/test_update_garmin_fit_devices.py as part of its steps; the tests exercise update_garmin_fit_devices.py functionality and CLI behavior and completed successfully in local CI validation.
• The updater's main() is exercised by tests, including writing a temporary settings.qml and fit_profile.hpp and verifying the --github-output changed=true/false behavior, which passed.
• Unit tests covering parsing, mapping consistency, exclusion rules, idempotence, and non-target QML regions were executed and succeeded.

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Codex Task