Pre-select routine weight units from the user profile (#2205)

[Dmao233]

Proposed Changes

When creating a SlotEntry or WorkoutLog without an explicit weight_unit, the API always stored kilograms because that is the model default. Users who set weight_unit to lb on their profile still got kg on new routine entries.

This PR defaults the unit from the owner's profile only on create:

• New slot entries use the routine owner's profile (kg → unit 1, lb → unit 2).
• New workout logs use the log owner's profile (REST user= and PowerSync user_id=).
• An explicit weight_unit in the request, including null, is kept.
• PATCH / edits do not rewrite the stored unit if the profile later changes.

The model default remains kg, so the serializer inspects initial_data instead of validated_data. Checking validated_data after DRF applies the model default cannot tell "client omitted the field" from "client sent kg".

Web/Flutter UI pre-selection still belongs in wger-project/react and the Flutter app. This repo only owns the API default used when a client omits the field.

Related Issue(s)

Fixes #2205

See also open PRs #2264 and #2439, which attempt the same feature in perform_create. Those stay open; this version avoids the model-default trap, covers workout logs (including PowerSync create), and adds the new-vs-edit cases from the issue discussion.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Code has been formatted to avoid unnecessary diffs (ruff format && isort .)
• If the feature is big enough or if there are manual steps needed (deployment changes etc.), write a small writeup in CHANGELOG.md

Testing

DJANGO_SETTINGS_MODULE=settings.ci uv run ./manage.py test \
  wger.manager.tests.test_api_weight_unit_default \
  wger.manager.tests.test_log_session_api \
  wger.manager.tests.test_api_ownership_checks \
  wger.manager.tests.test_weight_log \
  wger.manager.tests.test_copy_routine

8 new tests plus 39 related existing tests, all passing.