Fix variant count y-axis label

[Eldergenix]

Fixes #1784

Summary

• Updates the y-axis label on the non-synonymous coding variant count chart from Number of samples to Number of variants.
• Updates the matching StatsPage Jest snapshot.
• Leaves chart data and rendering logic unchanged.

Validation

• corepack pnpm@8.14.3 install --frozen-lockfile - passed; local Node v22.22.0 emitted the repository engine warning for ^18.17.1.
• corepack pnpm@8.14.3 jest --selectProjects browser --runTestsByPath browser/src/StatsPage/StatsPage.spec.tsx -u - passed; 1 test passed and 1 snapshot updated.
• corepack pnpm@8.14.3 eslint browser/src/StatsPage/StatsPage.tsx - passed.
• corepack pnpm@8.14.3 stylelint 'browser/src/StatsPage/StatsPage.tsx' - passed with existing Browserslist/stylelint deprecation warnings.
• corepack pnpm@8.14.3 ts-node ./browser/build/buildHelp.ts && corepack pnpm@8.14.3 exec tsc --noEmit - passed.
• corepack pnpm@8.14.3 jest --selectProjects browser - passed; 67 suites, 1765 tests, and 1144 snapshots.
• corepack pnpm@8.14.3 --dir browser build - passed; webpack reported existing asset/entrypoint size and Browserslist warnings.
• git diff --check - passed.

Notes

• Local validation used pinned pnpm 8.14.3 because pnpm 10 rejected the current lockfile format.
• A serial browser Jest attempt with --runInBand hit unrelated timeouts, so the CI-shaped browser project command above was used for full browser Jest validation.