Styling from design system

[anna-follestad-4ss]

What changed

• This PR adds a link to the 4subsea design repo: https://github.com/4Subsea/4subsea-design-system
• All fonts and colour definitions uses tokens from the 4subsea design system instead of the previous hard-coding.
• The design system is added through a pip install
• Added tests that check for hard-coded values (should generally be none)

Why

The styling was based on the current (July 2026) version of 4subsea styling, but hardcoded and so fixed to that version. We want the template repo to reflect actual latest version of the 4subsea style library, so that python dashboards are compliant with the company style.

Definition of done

CI covers the first two. See CONTRIBUTING.md for why each is here.

• pytest passes
• black is clean
• Every threshold, unit and label comes from a named constant, not a literal
• Anything machine-specific is in .env.example, not hardcoded (unless it's
  a documented, deliberate simplification - see README)
• Every page opened and looked at after the change
• At least one displayed number spot-checked against the source
• CLAUDE.md updated if this establishes a new convention, README.md if it
  changes how to run, refresh or deploy anything