Improve display of invoice details when information is missing

Update Avoirs page to display default text for missing invoice reference and amount for better user experience.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 9fcf4b21-eb0c-4e53-a567-e2ce4a6ad869
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/9fcf4b21-eb0c-4e53-a567-e2ce4a6ad869/mpVybiP

client/src/pages/Avoirs.tsx

@@ -450,7 +450,7 @@ export default function Avoirs() {
                 <div className="flex items-center justify-between">
                   <CardTitle className="text-lg flex items-center">
                     {getStatusIcon(avoir.status)}
-                    <span className="ml-2">#{avoir.invoiceReference}</span>
+                    <span className="ml-2">#{avoir.invoiceReference || 'Sans référence'}</span>
                   </CardTitle>
                   <Badge variant={getStatusVariant(avoir.status)}>
                     {avoir.status}
@@ -475,7 +475,7 @@ export default function Avoirs() {
                   
                   <div className="flex justify-between items-center">
                     <span className="text-sm text-muted-foreground">Montant:</span>
-                    <span className="font-bold text-lg">{avoir.amount.toFixed(2)} €</span>
+                    <span className="font-bold text-lg">{avoir.amount ? avoir.amount.toFixed(2) + ' €' : 'Non spécifié'}</span>
                   </div>
                   
                   {avoir.comment && (