docs: update ERROR_PAGES_README with recovery flows and drift test (#661)

[Ajibose]

Closes #661

Updates ERROR_PAGES_README.md to document the four current error surfaces, their recovery flows, and the actual CTA mappings used by the App Router error pages and transaction modal. Also adds a filesystem-based drift guard at src/app/__tests__/errorPagesDoc.test.ts so route additions or CTA label drift fail fast.

Implementation notes:

• Expanded the error-pages README with route behavior, retry flow details, environment notes, and pseudo-screenshots for reviewer clarity.
• Added a README cross-link so the main project docs point to the error recovery guide.
• Added a Vitest drift guard that verifies documented file paths exist, compares discovered error routes to the documented set, and checks the current CTA labels in the implementation.
• The focused guard passes with pnpm test errorPagesDoc.

Validation:

• pnpm test errorPagesDoc
• pnpm test currently reports unrelated pre-existing failures in other test files; the new error-pages test passes cleanly.

[vercel]

@Ajibose is attempting to deploy a commit to the 1nonly's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@Ajibose Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits