Unit ledger: allow posting a $0.00 charge

Relax the amount validation so a Charge/Expense/WriteOff can be posted with a
$0.00 amount (used as a note/memo on the ledger). Payments and prepayments
still require a positive amount; negatives remain blocked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/unit-profile/UnitLedgerTransactionForm.tsx

@@ -357,8 +357,15 @@ export default function UnitLedgerTransactionForm({ open, onOpenChange, unitId:
     }
 
     const numAmount = parseFloat(formData.amount);
-    if (!formData.amount || isNaN(numAmount) || numAmount <= 0) {
-      toast({ variant: "destructive", title: "Invalid Amount", description: "Please enter a valid positive amount." });
+    // Allow a $0.00 charge (used as a note/memo on the ledger). Payments and
+    // prepayments still require a positive amount.
+    const allowZero = ["Charge", "Expense", "WriteOff"].includes(formData.type);
+    if (isNaN(numAmount) || numAmount < 0 || (numAmount === 0 && !allowZero)) {
+      toast({
+        variant: "destructive",
+        title: "Invalid Amount",
+        description: allowZero ? "Please enter a valid amount ($0.00 is allowed)." : "Please enter a valid positive amount.",
+      });
       return;
     }