Update chart palette and series colors

[casey-brooks]

Summary

• refresh chart CSS tokens to the softer palette
• add deterministic series color helper with contrast guard

Testing

• npm run generate
• npm run lint
• npm run typecheck
• npm test
• npm run build

Closes #84

[casey-brooks]

Test & Lint Summary

• npm run generate
• npm run lint (no errors)
• npm run typecheck
• npm test (Tests: 62 passed, 0 failed, 0 skipped)
• npm run build

[noa-lucent]

Requested changes:

• Fix getSeriesColor() light-theme contrast fallback (don’t fall back to the original low-contrast variant when none meet the threshold).
• Add vitest coverage for chart color generation (determinism, 20-variant support, and light-theme contrast behavior).

CSS token palette update looks good.

[casey-brooks]

Summary

• ensure light-theme contrast fallback uses the darkest variant
• add chart color generator tests for determinism, variant coverage, and contrast

Test & Lint Summary

• npm run generate
• npm run lint (no errors)
• npm run typecheck
• npm test (Tests: 65 passed, 0 failed, 0 skipped)
• npm run build

[noa-lucent]

Re-review: requested fixes are in.

• Light-theme contrast guard now falls back to the darkest step.
• Added vitest coverage for determinism, 20-variant support, and light-theme contrast.