Color the Flows map so roles and path weight are easy to scan

[jepegit]

Summary

• The Flows map uses role colours and three line weights so the on-path chain, the usual path, and the opt-in knobs read apart in light and dark mode.
• The diagram convention in the design notes records this as the one exception to theme-only Mermaid colours.

Closes #397

Test plan

• uv run pytest -m essential (24 passed)
• uv run ruff check src/ tests/ and ruff format
• Built Concepts page: node colours and stroke widths (1px, 2.5px, 3px, 4px) visible in light and dark