feat: add tax withholding explanations and off-cycle disclaimer note

[krisxcrash]

Summary

• Adds an optional taxedAsDescription field to WageTypeGroup, rendered as supporting text under the derived "taxed as" value in OffCycleTaxWithholdingTable (mirrors the existing description field already shown under the wage-type label).
• Adds per-category "taxed as" explanation copy (regular/supplemental/reimbursement) to the Payroll.OffCycleTaxWithholding i18n namespace and wires it into OffCycleCreationPresentation's wageTypeGroups.
• Surfaces an off-cycle payroll tax-impact disclaimer above the tax withholding table in OffCycleCreationPresentation, using the same copy already shown in the existing off-cycle payroll UI (gws-flows), rendered via <Trans> for the inline-bold segment.

There was previously no way for a OffCycleTaxWithholdingTable consumer to explain how a wage type category is taxed, and no disclaimer that off-cycle payrolls can affect tax withholding when relied on exclusively.

Test plan

• npm run test -- --run — full suite passes (3824 passed, 1 pre-existing expected fail)
• npx tsc --noEmit — no errors
• npm run lint — 0 errors, only pre-existing warnings
• npm run i18n:generate — regenerated src/i18n/types.d.ts and committed
• Added/updated tests in OffCycleTaxWithholdingTable.test.tsx and OffCycleCreation.test.tsx
• Updated OffCycleTaxWithholdingTable.stories.tsx with the new taxedAsDescription copy

🤖 Generated with Claude Code