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>

artifacts/voix-du-peuple/src/pages/home.tsx

@@ -18,6 +18,7 @@ import {
   addExtraHeader,
   removeExtraHeader,
   getVisitorId,
+  ApiError,
 } from "@workspace/api-client-react";
 import { ConsentDialog } from "@/components/consent-dialog";
 import { Button } from "@/components/ui/button";
@@ -154,8 +155,14 @@ export default function Home() {
         queryClient.invalidateQueries({ queryKey: getListIdeasQueryKey() });
         queryClient.invalidateQueries({ queryKey: getGetIdeaStatsQueryKey() });
       },
-      onError: () => {
-        setSubmitResult({ success: false, message: "Une erreur est survenue lors de l'envoi. Veuillez réessayer." });
+      onError: (error) => {
+        const apiMessage = error instanceof ApiError
+          ? (error.data as { message?: string } | null)?.message
+          : undefined;
+        setSubmitResult({
+          success: false,
+          message: apiMessage ?? "Une erreur est survenue lors de l'envoi. Veuillez réessayer.",
+        });
       },
       onSettled: () => {
         removeExtraHeader("x-hcaptcha-token");
@@ -411,7 +418,11 @@ export default function Home() {
                 : <Info className="h-4 w-4 text-amber-600" />
               }
               <AlertTitle>
-                {submitResult.success ? "Contribution enregistrée" : "Contribution non retenue"}
+                {submitResult.success
+                  ? "Contribution enregistrée"
+                  : submitResult.reason
+                    ? "Contribution non retenue"
+                    : "Envoi impossible"}
               </AlertTitle>
               <AlertDescription>
                 {submitResult.message}

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";