feat: allow selecting the default variant

[zerone0x]

Summary

• add a default entry to the variant picker
• store the default selection as nil so saved model variant overrides are cleared
• keep the UI label as default while applying the underlying nil variant value

Verification

• git diff --check
• ./run_tests.sh -t unit (blocked: nvim: command not found in this environment)

Fixes #426

[sudo-tee]

Thanks for the PR.

Looks good to me :)