Fix error when setting monetary amount to zero or undefined

Update Avoirs page component to correctly handle monetary input values, allowing amounts to be set to zero or undefined by parsing input strings to floats.

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/Ii96ETr

client/src/pages/Avoirs.tsx

@@ -1362,7 +1362,8 @@ export default function Avoirs() {
                         step="0.01" 
                         placeholder="Montant en euros (optionnel)" 
                         {...field}
-                        value={field.value || ""}
+                        value={field.value ?? ""}
+                        onChange={(e) => field.onChange(e.target.value ? parseFloat(e.target.value) : undefined)}
                       />
                     </FormControl>
                     <FormMessage />