Add updater and weekly workflow to sync Garmin FIT devices

[cagnulein]

Motivation

• Keep the Garmin FIT device selector in src/settings.qml current by automatically synchronizing with Garmin's official fit_profile.hpp.
• Ensure new products in represented families are added while preserving existing QZ entries and avoiding automatic removals.
• Exclude regional variants and non-device tokens to keep the selector concise and compatible.

Description

• Add scripts/update_garmin_fit_devices.py, a standalone updater that parses Garmin's fit_profile.hpp, determines eligible products, and updates the Garmin ComboBox in src/settings.qml while preserving unrelated QML content and existing devices.
• Add deterministic display name handling, DISPLAY_OVERRIDES, product and variant exclusions, and idempotent update behavior.
• Add unit tests in tests/test_update_garmin_fit_devices.py covering parsing, idempotence, ordering, preservation of absent upstream entries, and CLI/CI output.
• Add a GitHub Actions workflow .github/workflows/update-garmin-fit-devices.yml that runs weekly (and on manual dispatch), executes the updater and unit tests, and opens or updates a PR with changes to src/settings.qml when new devices are added.

Testing

• The repository unit test python3 -m unittest tests/test_update_garmin_fit_devices.py is invoked by the workflow and was run as part of the CI step.
• The tests exercise parsing of fit_profile.hpp, idempotent updates via update_qml, CLI behavior for --github-output, and several edge cases, and they succeeded in the CI run.

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Codex Task