feat: implement MobaXterm-style layout (Phase 1-2-4)

实现 MobaXterm 风格的界面重构,包含会话树、工作区面板和分屏功能。

新增功能:
- 左侧会话树支持文件夹分组和展开/折叠
- 工作区垂直分屏(终端 + SFTP)
- 可拖拽调整分割比例
- 状态持久化到 localStorage
- 顶部工具栏(样式占位)

技术实现:
- 新增 sessionTreeStore 和 workspaceStore 状态管理
- 新增 SessionTree/SessionTreeNode 递归组件
- 新增 SplitPane 可拖拽分割组件
- 重构 SftpPanel 为 props 驱动
- 新增 MobaLayout 主布局
- 路由默认重定向到 /moba

依赖更新:
- 安装 @vueuse/core 用于拖拽功能

待实现:
- Phase 3: 会话树拖拽排序
- Phase 5: 数据迁移
- Phase 6: 快捷键、右键菜单、搜索等优化

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package-lock.json

@@ -8,6 +8,7 @@
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         "@xterm/addon-attach": "^0.12.0",
         "@xterm/addon-fit": "^0.10.0",
         "axios": "^1.13.4",
@@ -1004,6 +1005,12 @@
         "undici-types": "~7.16.0"
       }
     },
+    "node_modules/@types/web-bluetooth": {
+      "version": "0.0.21",
+      "resolved": "https://registry.npmmirror.com/@types/web-bluetooth/-/web-bluetooth-0.0.21.tgz",
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       "resolved": "https://registry.npmjs.org/@vitejs/plugin-vue/-/plugin-vue-6.0.4.tgz",
@@ -1245,6 +1252,44 @@
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+      "resolved": "https://registry.npmmirror.com/@vueuse/core/-/core-14.2.1.tgz",
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+        "@vueuse/metadata": "14.2.1",
+        "@vueuse/shared": "14.2.1"
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+      "funding": {
+        "url": "https://github.com/sponsors/antfu"
+      },
+      "peerDependencies": {
+        "vue": "^3.5.0"
+      }
+    },
+    "node_modules/@vueuse/metadata": {
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+      "license": "MIT",
+      "funding": {
+        "url": "https://github.com/sponsors/antfu"
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+      "resolved": "https://registry.npmmirror.com/@vueuse/shared/-/shared-14.2.1.tgz",
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       "resolved": "https://registry.npmjs.org/@xterm/addon-attach/-/addon-attach-0.12.0.tgz",