Show API error message to user instead of generic fallback

- Export ApiError from api-client-react package
- home.tsx onError: extract error.data.message from ApiError and display it
  (cooldown, rate limit, consent_required, validation messages are now visible)
- Differentiate alert title: "Contribution non retenue" for AI rejection
  (has a reason), "Envoi impossible" for technical errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/api-client-react/src/index.ts

@@ -7,5 +7,6 @@ export {
   getVisitorId,
   addExtraHeader,
   removeExtraHeader,
+  ApiError,
 } from "./custom-fetch";
 export type { AuthTokenGetter } from "./custom-fetch";