fix(ui): add disabled state and feedback to usage tips Add button

The Add button silently returned when no parameter or value was
provided, looking clickable but doing nothing. Keep it disabled until
both inputs are filled, validate the numeric value, surface save
failures via toast without clearing user input, and confirm additions
vs overwrites with success toasts. Includes translations for all
locales.

locales/zh-TW.json

@@ -1593,7 +1593,11 @@
                 "clipSkip": "Clip Skip",
                 "valuePlaceholder": "數值",
                 "add": "新增",
-                "invalidRange": "無效的範圍格式。請使用 x.x-y.y"
+                "invalidRange": "無效的範圍格式。請使用 x.x-y.y",
+                "invalidValue": "請輸入有效的數值",
+                "saveFailed": "儲存預設參數失敗",
+                "added": "已新增預設參數",
+                "updated": "已更新預設參數"
             },
             "triggerWords": {
                 "label": "觸發詞",