Deduplicate CSV export logic

[Eldergenix]

Fixes #1777

Summary

• Add a shared CSV export helper for table-style downloads.
• Reuse that helper from the copy number, mitochondrial, structural, and small variant export buttons.
• Keep each export button's dataset-specific column definitions local.
• Add focused tests for the shared serializer and filename formatting while preserving the existing CSV output behavior.

Validation

• corepack pnpm@8.14.3 install --frozen-lockfile - passed; no lockfile changes. The local shell used Node v22.22.0 and pnpm warned that the repo declares Node ^18.17.1.
• corepack pnpm@8.14.3 jest --selectProjects browser --runTestsByPath browser/src/exportTableToCsv.spec.ts browser/src/VariantList/ExportVariantsButton.spec.tsx --runInBand --silent - passed, 2 suites and 44 tests.
• corepack pnpm@8.14.3 ts-node ./browser/build/buildHelp.ts && corepack pnpm@8.14.3 typecheck - passed.
• corepack pnpm@8.14.3 eslint browser - passed.
• corepack pnpm@8.14.3 stylelint 'browser/**/*.(js|jsx|ts|tsx)' - passed with existing Browserslist freshness and unit-whitelist deprecation warnings.
• corepack pnpm@8.14.3 jest --selectProjects browser --runInBand --silent - passed, 68 suites, 1771 tests, and 1144 snapshots.
• corepack pnpm@8.14.3 --dir browser build - passed with existing webpack asset and entrypoint size warnings.
• git diff --check - passed.

Notes

• The shared serializer intentionally preserves the previous CSV formatting behavior so this PR stays a deduplication refactor.
• I did not run a manual browser export smoke test; validation is from focused serializer coverage, existing export button coverage, full browser tests, typecheck, lint, stylelint, and build.