feat: add value editor and Set NULL in cell detail panel

Double-click value area or click Edit button to open textarea editor
for long text/JSON. Add Set NULL button for quick null assignment.
Fix: null values via ?? operator were ignored in dirty row display.

src/i18n/locales/en.ts

@@ -214,6 +214,8 @@ export default {
     noComment: "No comment",
     formattedJson: "Formatted JSON",
     copyValue: "Copy Value",
+    editValue: "Edit Value",
+    setNull: "Set NULL",
     copyColumnName: "Copy Column Name",
     copySqlCondition: "Copy SQL Condition",
     transpose: "Transpose Row",

src/i18n/locales/zh-CN.ts

@@ -213,6 +213,8 @@ export default {
     noComment: "暂无注释",
     formattedJson: "格式化 JSON",
     copyValue: "复制值",
+    editValue: "编辑值",
+    setNull: "设为 NULL",
     copyColumnName: "复制列名",
     copySqlCondition: "复制 SQL 条件",
     transpose: "转置查看",