feat(merge): 添加合并预览 Tab 页

- 新增第三个 Tab「合并预览」，只读展示系统+用户 PATH 完整列表
- 选项卡切换时自动刷新合并列表
- 支持无效/重复路径高亮
- 新增中英文翻译

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lua/config.lua

@@ -51,6 +51,7 @@ local config = {
         search_placeholder = "Search...",
         tab_sys = "System Variables",
         tab_user = "User Variables",
+        tab_merged = "Merged Preview",
         export_title = "Export PATH",
         import_title = "Import PATH"
     },