feat: offline gains — courbe inversée 2h, cap 25%, écran résumé

offlineEfficiency() : 100% (0-15min) → 25% (1h) → 0% (2h).
computeOfflineGains() intègre la courbe par tranches de 1min.
GameState.lastOnline ajouté, store hydrate avec offline report.
OfflineReport.tsx affiché au retour si absence > 60s.
13 nouveaux tests (66 total, tous passent).

Frontend/src/components/OfflineReport.tsx

@@ -0,0 +1,65 @@
+// OfflineReport.tsx — Écran "Pendant ton absence..." affiché au retour offline
+
+import { useGameStore } from "../store/useGameStore";
+import { formatNumber } from "../utils/formatNumber";
+
+function formatDuration(ms: number): string {
+  const minutes = Math.floor(ms / 60_000);
+  if (minutes < 60) return `${minutes}min`;
+  const hours = Math.floor(minutes / 60);
+  const remainMinutes = minutes % 60;
+  return remainMinutes > 0 ? `${hours}h${remainMinutes}min` : `${hours}h`;
+}
+
+export function OfflineReport() {
+  const report = useGameStore((s) => s.offlineReport);
+  const dismiss = useGameStore((s) => s.dismissOfflineReport);
+
+  if (!report || !report.wasOffline) return null;
+
+  const effPercent = Math.round(report.efficiency * 100);
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+      <div className="gp max-w-sm w-full mx-4 text-center">
+        <span className="gp-title text-lg!">Pendant ton absence...</span>
+
+        <div className="flex flex-col gap-3 mt-2">
+          <div className="flex justify-between">
+            <span className="gp-label">Durée</span>
+            <span className="gp-value">{formatDuration(report.duration)}</span>
+          </div>
+
+          <div className="flex justify-between">
+            <span className="gp-label">Efficacité marais</span>
+            <span className={`gp-value ${effPercent > 50 ? "gp-accent-green" : "gp-accent-amber"}`}>
+              {effPercent}%
+            </span>
+          </div>
+
+          <div className="gp-sep" />
+
+          <div className="flex justify-between items-center">
+            <span className="gp-label">Têtards récoltés</span>
+            <span className="gp-value gp-accent-green text-lg!">
+              +{formatNumber(report.gains)}
+            </span>
+          </div>
+
+          {report.efficiency < 0.5 && (
+            <p className="gp-label text-center">
+              Le marais s'endort sans toi... Joue activement pour maximiser ta production !
+            </p>
+          )}
+        </div>
+
+        <button
+          onClick={dismiss}
+          className="gp-btn gp-btn--buy w-full mt-3 py-2!"
+        >
+          Retour au marais
+        </button>
+      </div>
+    </div>
+  );
+}