Improve handling of invoice amounts to ensure correct data processing

Refactor the `amount` field validation in `Avoirs.tsx` to use a Zod transform for robust string-to-number conversion and validation, handling empty, null, undefined, NaN, and non-positive values.

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/KZQgDPI

client/src/pages/Avoirs.tsx

@@ -71,10 +71,19 @@ const avoirSchema = z.object({
   supplierId: z.coerce.number().min(1, "Veuillez sélectionner un fournisseur"),
   groupId: z.coerce.number().min(1, "Veuillez sélectionner un magasin"),
   invoiceReference: z.string().optional(),
-  amount: z.union([
-    z.string().length(0).transform(() => undefined),
-    z.coerce.number().positive()
-  ]).optional(),
+  amount: z.string().transform((val) => {
+    if (val === "" || val === null || val === undefined) {
+      return undefined;
+    }
+    const num = parseFloat(val);
+    if (isNaN(num)) {
+      throw new Error("Montant invalide");
+    }
+    if (num <= 0) {
+      throw new Error("Le montant doit être supérieur à 0");
+    }
+    return num;
+  }).optional(),
   comment: z.string().optional(),
   commercialProcessed: z.boolean().optional(),
   status: z.enum(["En attente de demande", "Demandé", "Reçu"]).optional(),