Return insufficient_wallet when Transfer billing fails for zero balance.

Expose a stable error_code on chunked upload finalize so webmail can prompt wallet top-up instead of a generic upload failure.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/Support/TransferWalletErrors.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace App\Support;
+
+use RuntimeException;
+
+class TransferWalletErrors
+{
+    public const CODE_INSUFFICIENT_WALLET = 'insufficient_wallet';
+
+    public static function isInsufficientBalance(RuntimeException $e): bool
+    {
+        $message = strtolower($e->getMessage());
+
+        return str_contains($message, 'insufficient wallet')
+            || str_contains($message, 'top up');
+    }
+}