fix: 修复侧边栏拖拽区域的国际化文案和鼠标样式

- 添加 toggleSidebar 的中英文翻译
- 移除无效的拖拽鼠标样式，改为 pointer
- 使用 i18n 替换硬编码的英文文本

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/plugins/i18n/en.json

@@ -77,7 +77,8 @@
     "admin": "Admin",
     "agent": "Agent",
     "basicPlan": "Basic Plan",
-    "normal": "Normal"
+    "normal": "Normal",
+    "toggleSidebar": "Toggle Sidebar"
   },
   "premium": {
     "premium": "premium",

frontend/src/plugins/i18n/zh.json

@@ -77,7 +77,8 @@
     "admin": "管理员",
     "agent": "代理商",
     "basicPlan": "基础版",
-    "normal": "正常"
+    "normal": "正常",
+    "toggleSidebar": "切换侧边栏"
   },
   "premium": {
     "premium": "会员计划",