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/src/layouts/MobaLayout.vue

@@ -0,0 +1,36 @@
+<script setup lang="ts">
+import { onMounted } from 'vue'
+import { useSessionTreeStore } from '../stores/sessionTree'
+import { useWorkspaceStore } from '../stores/workspace'
+import TopToolbar from '../components/TopToolbar.vue'
+import SessionTree from '../components/SessionTree.vue'
+import WorkspacePanel from '../components/WorkspacePanel.vue'
+
+const treeStore = useSessionTreeStore()
+const workspaceStore = useWorkspaceStore()
+
+onMounted(() => {
+  treeStore.restore()
+  workspaceStore.restore()
+
+  if (treeStore.nodes.length === 0) {
+    treeStore.initFromConnections()
+  }
+})
+</script>
+
+<template>
+  <div class="h-screen flex flex-col bg-slate-950 text-slate-100">
+    <TopToolbar />
+
+    <div class="flex-1 min-h-0 flex">
+      <div class="w-72 flex-shrink-0">
+        <SessionTree />
+      </div>
+
+      <div class="flex-1 min-w-0">
+        <WorkspacePanel />
+      </div>
+    </div>
+  </div>
+</template>