test: add withholding tax process guides

[spc-agent-00]

Summary

• add end-to-end Playwright guides for purchase and sales withholding tax payment flows
• capture stable visual snapshots for use as documentation image sources
• add a test-only Frappe factory API fixture and shared screenshot helpers
• centralize Desk date formatting and document-name normalization

Test plan

• CI=1 yarn playwright test specs/guides/purchase-wht-process.spec.ts specs/guides/sales-wht-process.spec.ts --project=chromium
• pre-commit run