[codex] Improve Design Studio lineage

[kevrez]

What changed

• Reworked Design Studio into a Design Library that collapses edit lineage into one titled row with v1/v2/v3 tracking and expandable per-version history.
• Added shared design titles, title persistence, custom dimensions/aspect-ratio inputs, and full prompt previews for create/edit flows.
• Added request parsing for title and dimensions, title update support, and custom-size OpenAI cost estimates.
• Added an APP_ENV=test guard that rejects OPENAI_MODE=real so local mock smoke tests cannot inherit paid OpenAI settings from .env by accident.
• Updated TODO.md to reflect the Design Studio and OpenAI test-mode safety changes.

Root cause

Settings() loads .env by default, so starting a temporary app with APP_ENV=test did not override OPENAI_MODE=real. A smoke test that looked mocked could still construct the real OpenAI provider.

Validation

• uv run pytest tests/unit/test_config.py -q
• uv run pytest -q (99 passed, 3 skipped because TEST_DATABASE_URL is unset)
• uv run ruff check on touched files
• uv run mypy src/ecommerce_agent/api/routes.py src/ecommerce_agent/config.py src/ecommerce_agent/services/openai_images.py
• In-app browser smoke of Design Studio lineage, prompt preview, aspect ratio controls, and expanded history