phsl / new-api

Unify the setup initialization endpoint’s error contract to match the rest
of the project and keep the frontend unchanged.

Changes
- controller/setup.go: Return HTTP 200 with {success:false, message} for all
  predictable errors in POST /api/setup, including:
  - already initialized
  - invalid payload
  - username too long
  - password mismatch
  - password too short
  - password hashing failure
  - root user creation failure
  - option persistence failures (SelfUseModeEnabled, DemoSiteEnabled)
  - setup record creation failure
- web/src/components/setup/SetupWizard.jsx: Restore catch handler to the
  previous generic toast (frontend logic unchanged).
- web/src/helpers/utils.jsx: Restore the original showError implementation
  (no Axios response.data parsing required).

Why
- Keep API behavior consistent across endpoints so the UI can rely on the
  success flag and message in the normal .then() flow instead of falling
  into Axios 4xx errors that only show a generic "400".

Impact
- UI now displays specific server messages during initialization without
  frontend adaptations.
- Note: clients relying solely on HTTP status codes for error handling
  should inspect the JSON body (success/message) instead.

No changes to the happy path; initialization success responses are unchanged.