♦fix: handle network errors in validateAndSignUpUser (#4285)

[PS01K]

Issue:

Fixes #4285

When submitting the signup form during a network error, request timeout, or server disconnection, Axios produces an error object where error.response is undefined.

Previously, validateAndSignUpUser() attempted to access response.data.error directly inside its .catch() handler. This caused a synchronous TypeError: Cannot read properties of undefined (reading 'data'), preventing resolve({ error }) from executing. Consequently, the promise returned to React Final Form remained pending indefinitely, freezing the signup form in a permanent submitting state with disabled submit controls and no user feedback.

Changes:

• client/modules/User/actions.ts:
  • Safely extract error messages in validateAndSignUpUser using optional chaining (error.response?.data?.error || error.response?.data?.message || error.message || 'Unknown error.') so authError is dispatched and resolve({ error }) is always reached.
  • Updated authError(error: Error | string) type definition to permit string error payloads.
  • Guarded against similar missing error.response crashes in validateAndLoginUser, logoutUser, unlinkService, and setUserCookieConsent.
  • Returned the promise from logoutUser thunk.
• client/modules/User/actions.unit.test.ts:
  • Added unit test suite covering successful signup, API 422 error handling, network error fallbacks (ensuring the promise resolves and does not hang), and login/logout error handling.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #4285
• meets the standards outlined in the accessibility guidelines

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