Add synthesis refresh countdown display

Show a live countdown (15→0s) in the synthesis header indicating when the
next auto-refresh will occur. Resets on every successful synthesis fetch.

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

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

@@ -134,6 +134,14 @@ export default function Home() {
     query: { refetchInterval: 15000 },
   });
 
+  const [countdown, setCountdown] = React.useState(15);
+  React.useEffect(() => { setCountdown(15); }, [synthesis]);
+  React.useEffect(() => {
+    if (countdown <= 0) return;
+    const t = setTimeout(() => setCountdown(c => c - 1), 1000);
+    return () => clearTimeout(t);
+  }, [countdown]);
+
   // Soumission effective après confirmation du consentement (ou si déjà consenti)
   const doActualSubmit = (data: SubmitIdeaValues) => {
     if (captchaToken) {
@@ -544,7 +552,7 @@ export default function Home() {
               <Users className="h-3.5 w-3.5" /> Synthèse des contributions
             </h2>
             <p className="text-xs text-muted-foreground mt-0.5">
-              Mise à jour à chaque nouvelle contribution
+              Mise à jour à chaque nouvelle contribution · <span className="font-mono">{countdown}s</span>
             </p>
           </div>
           <div className="flex items-center gap-2 flex-shrink-0">