Fix theme-aware chart colour selection

[Bjorkan]

AI disclosure: This entire pull request was written by OpenAI GPT-5.

What this PR does

Replaces the fixed chart-series token order and RGB blends with a categorical
palette derived from the active theme. Some themes reuse semantic colours — for
example, Phosphor and Radar use the same value for primary and green — which
made unrelated chart series identical or nearly indistinguishable.

• Uses OKLCH distance and background contrast to keep categorical series distinct.
• Reserves green, yellow, and red colour families for success, warning, and error semantics.
• Uses primary for neutral single-series charts, generated colours for categories and comparisons, and primary-derived ramps for sequential data.
• Validates contrast and colour separation across all 18 bundled themes.

Type of change

• Bug fix
• New feature
• Refactor
• Tests
• Docs / config
• Other:

Checklist

• npm run build passes (the real typecheck)
• npm run lint passes
• npm test passes
• New logic has tests (TDD for bugfixes and features)
• Frontend types still match the beacon-server contract
• I have read CONTRIBUTING.md

Testing notes

• npm test — 1206 passed
• npm run build
• npm run lint — 0 errors, 2 pre-existing warnings
• Verified the generated palette against every bundled dark and light theme.

Reviewers should pay particular attention to multi-series charts in the
Phosphor, Radar, Amber, and Earth themes, which previously contained duplicate
semantic colours.