fix(sidebar): 侧边栏四项反馈（docs/17）

空状态按钮对齐页签缩进；启动自动恢复上次工作区（prefs 存 lastWorkspace、
启动校验存在性）；空文件夹展开显示「（空）」、文件夹行加粗（展开逻辑经
组件测试证实本就可用）；新建泛化到任意文件夹：树头＋文档/＋文件夹、
文件夹右键在此新建文档/文件夹。vitest 86 通过 + 2 跳过。

tests/sidebar-tree.test.tsx

@@ -0,0 +1,101 @@
+// 文件树回归：子文件夹展开、空文件夹提示、文件夹右键新建文档/文件夹。
+import { describe, it, expect, vi } from 'vitest'
+import { createRoot } from 'react-dom/client'
+import { act } from 'react-dom/test-utils'
+
+vi.mock('../src/services/platform', async (importOriginal) => {
+  const orig = await importOriginal<typeof import('../src/services/platform')>()
+  return {
+    ...orig,
+    listDir: async (dir: string) => {
+      if (dir === 'C:/ws') {
+        return [
+          { name: 'sub', path: 'C:/ws/sub', isDir: true },
+          { name: 'empty', path: 'C:/ws/empty', isDir: true },
+          { name: 'a.md', path: 'C:/ws/a.md', isDir: false },
+        ]
+      }
+      if (dir === 'C:/ws/sub') return [{ name: 'b.md', path: 'C:/ws/sub/b.md', isDir: false }]
+      return []
+    },
+  }
+})
+
+import Sidebar from '../src/components/Sidebar'
+
+const sleep = (ms: number) => new Promise((r) => setTimeout(r, ms))
+
+async function renderSidebar(extra: Record<string, unknown> = {}) {
+  const container = document.createElement('div')
+  document.body.appendChild(container)
+  const root = createRoot(container)
+  await act(async () => {
+    root.render(
+      <Sidebar
+        mode="files"
+        onModeChange={() => undefined}
+        workspaceRoot="C:/ws"
+        activePath={null}
+        onOpenFile={() => undefined}
+        onOpenFolder={() => undefined}
+        outline={[]}
+        onJumpHeading={() => undefined}
+        tauriMode
+        {...extra}
+      />,
+    )
+  })
+  await act(async () => { await sleep(30) })
+  return { container, root }
+}
+
+function findButton(container: HTMLElement, text: string) {
+  return [...container.querySelectorAll('button')].find((b) => b.textContent?.includes(text))
+}
+
+describe('文件树', () => {
+  it('子文件夹点击展开能看到下级文件', async () => {
+    const { container, root } = await renderSidebar()
+    await act(async () => {
+      findButton(container, 'sub')?.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    })
+    await act(async () => { await sleep(30) })
+    expect(container.innerHTML).toContain('b.md')
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+
+  it('空文件夹展开后有（空）提示而不是空白', async () => {
+    const { container, root } = await renderSidebar()
+    await act(async () => {
+      findButton(container, 'empty')?.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    })
+    await act(async () => { await sleep(30) })
+    expect(container.innerHTML).toContain('（空）')
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+
+  it('文件夹右键菜单有新建文档/文件夹并回传该文件夹', async () => {
+    const created: string[] = []
+    const folders: string[] = []
+    const { container, root } = await renderSidebar({
+      onCreateFileIn: (dir: string) => created.push(dir),
+      onCreateFolderIn: (dir: string) => folders.push(dir),
+    })
+    await act(async () => {
+      findButton(container, 'sub')?.dispatchEvent(new MouseEvent('contextmenu', { bubbles: true }))
+    })
+    await act(async () => { await sleep(10) })
+    const fileItem = findButton(container, '在此新建文档')
+    const folderItem = findButton(container, '在此新建文件夹')
+    expect(fileItem).toBeTruthy()
+    expect(folderItem).toBeTruthy()
+    await act(async () => {
+      fileItem?.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    })
+    expect(created).toEqual(['C:/ws/sub'])
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+})