Remove stale Gemma default wording

[ResearchForumOnline]

Summary

• remove residual UI/manual copy that still described Gemma as the OpenZero default
• describe offline bundles generically as carrying the selected local Ollama model store
• keep Gemma E4B explicitly compatibility-only and Ministral Q5 the verified default
• add a regression test for retired default copy

Verification

• python -m unittest tests.test_model_catalog_contract -v
• python -m unittest discover -s tests -p test_*.py -q (77 passed)
• python -m py_compile brain/app.py
• git diff --check