fix(cookbook): discover macOS HuggingFace cache location (#5978)

[Spoofiecus]

Summary

The cookbook local-cache scanner (routes/cookbook_helpers.py, _cached_model_scan_script) only listed ~/.cache/huggingface/hub (Linux) and never consulted huggingface_hub, so on macOS models cached at the platform default ~/Library/Caches/huggingface/hub were invisible in the model picker.

Target branch

• This PR targets dev, not main. All PRs land in dev; main is curated by the maintainer at each release. If your PR is on main by accident, click "Edit" on this PR and change the base.

Linked Issue

Fixes #5978

Type of Change

• Bug fix (non-breaking - fixes a confirmed issue)

Checklist

• I searched open issues and open PRs - this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above - no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.

How to Test

1. cd '/home/user/Odysseus Project' && ./venv/bin/python -m pytest tests/test_cookbook_helpers.py -k 'cached or hf_cache or platform or 5978' -v -> 13 passed, 1 skipped (windows-only).
2. Live: dev server :7001 restarted with the new code; /api/health healthy; original :7000 untouched.

Runtime attestation

• py_compile + focused pytest: 13 passed / 1 skipped.
• Dev :7001 booted with the new code (uvicorn app:app); /api/health healthy; original :7000 untouched.
• macOS host not available (box is Linux); the darwin branch is covered by a behavioral test that execs the shipped scanner def under sys.platform == 'darwin'.
• Pre-existing 4-test baseline failures (rule 30): 3x test_run_focus dry-run space-in-path exact-match + 1x test_opencode_setup_provider_aliases_resolve drift. Not caused by this PR.

Visual / UI changes

None - no static/js, CSS, HTML, SVG, or DOM rendering changed.