feat: 新增商品 Excel 匯入功能與修復 HTTPS 混合內容問題

1. 新增商品 Excel 匯入功能 (ProductImport, Export Template)
2. 調整商品代號驗證規則為 1-5 碼 (Controller & Import)
3. 修正 HTTPS Mixed Content 問題 (AppServiceProvider)

composer.json

@@ -13,6 +13,7 @@
         "inertiajs/inertia-laravel": "^2.0",
         "laravel/framework": "^12.0",
         "laravel/tinker": "^2.10.1",
+        "maatwebsite/excel": "^3.1",
         "spatie/laravel-activitylog": "^4.10",
         "spatie/laravel-permission": "^6.24",
         "stancl/jobpipeline": "^1.8",
@@ -93,4 +94,4 @@
     },
     "minimum-stability": "stable",
     "prefer-stable": true
-}
+}