fix: safe JSON parse in checkout handlers, show proper error on 500

frontend/src/app/pricing/page.tsx

@@ -378,12 +378,16 @@ export default function PricingPage() {
           billing_period: isYearly ? "yearly" : "monthly",
         }),
       });
-      const data = await res.json();
+
+      // Safe JSON parse — backend may return plain text on 500
+      let data: any = {};
+      try { data = await res.json(); } catch { /* ignore */ }
+
       const url = data.data?.url ?? data.url;
       const backendMessage = data.message ?? data.error ?? data.data?.error;
 
       if (!res.ok) {
-        throw new Error(backendMessage || t('pricing.toast.paymentError'));
+        throw new Error(backendMessage || `Erreur serveur ${res.status}`);
       }
 
       if (url) {
@@ -420,10 +424,14 @@ export default function PricingPage() {
         },
         body: JSON.stringify({ package_index: packageIndex }),
       });
-      const data = await res.json();
+
+      // Safe JSON parse
+      let data: any = {};
+      try { data = await res.json(); } catch { /* ignore */ }
+
       const url = data.data?.url ?? data.url;
       if (!res.ok) {
-        throw new Error(data.message ?? data.error ?? t('pricing.toast.paymentError'));
+        throw new Error(data.message ?? data.error ?? `Erreur serveur ${res.status}`);
       }
       if (url) {
         window.location.replace(url);