fix: update low stock warning threshold to 20% of max_stock

1. 移除硬編碼的門檻值 5。
2. 改用 stock <= (max_stock * 0.2) 進行動態判斷。
3. 增加 max_stock > 0 過濾，避免空貨道產生錯誤警示。

app/Http/Controllers/Admin/RemoteController.php

@@ -207,7 +207,8 @@ class RemoteController extends Controller
         $machineQuery = Machine::withCount([
             'slots as slots_count',
             'slots as low_stock_count' => function ($query) {
-                $query->where('stock', '<=', 5);
+                $query->where('max_stock', '>', 0)
+                    ->whereRaw('stock <= (max_stock * 0.2)');
             },
             'slots as expiring_soon_count' => function ($query) {
                 $query->whereNotNull('expiry_date')