feat: add import directory button to sidebar header

[Roganic]

Summary

Adds a folder icon button in the top-left sidebar header (next to the MD Viewer logo) that opens the existing "Add Workspace" dialog.

Before: the only way to import a directory was to switch to the workspace view (焦点/全量 tab) and find the + button there — not obvious for new users.

After: a small folder icon sits in the header, always visible regardless of which sidebar tab is active.

Changes

• src/client/html.ts: add .import-dir-btn button inside <h1>, calls showAddWorkspaceDialog() which is already exposed on window by sidebar-workspace.ts
• src/client/css.ts: style the button — 26×26px, transparent background, hover state consistent with existing toolbar icon buttons; margin-left: auto pushes it to the far right of the header row

No new logic or API changes — the dialog, path autocomplete, and workspace scan flow are fully reused.

Test plan

• Click the folder icon in the sidebar header → "添加工作区" dialog opens
• Works in all three sidebar tabs (焦点 / 全量 / 列表)
• Enter a directory path, confirm → workspace is added and sidebar switches to workspace view
• Button hover state looks consistent with other icon buttons

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces an "Import Directory" button to the MD Viewer header, including its HTML structure and CSS styling. The review feedback focuses on improving the button's visual consistency with existing UI elements by adjusting the border radius and transitions, as well as enhancing accessibility for screen readers by adding an aria-label.

[gemini-code-assist]

For consistency with other interactive elements in this repository, the border-radius should be 6px and the transition should explicitly use background-color with the ease timing function. Additionally, adding an :active state with a slight scale effect would match the behavior of other buttons in the UI (like the copy buttons).

    .import-dir-btn {
      margin-left: auto;
      display: flex;
      align-items: center;
      justify-content: center;
      width: 26px;
      height: 26px;
      padding: 0;
      border: none;
      border-radius: 6px;
      background: transparent;
      color: #57606a;
      cursor: pointer;
      flex-shrink: 0;
      transition: background-color 0.15s ease, color 0.15s ease;
    }
    .import-dir-btn:hover {
      background: #e8eaed;
      color: #24292e;
    }
    .import-dir-btn:active {
      transform: scale(0.95);
    }

[gemini-code-assist]

To ensure accessibility for screen readers and maintain consistency with other icon-only buttons in the sidebar (such as the annotation density and filter toggles), please add an aria-label attribute to this button.

Suggested change

	<button
	class="import-dir-btn"
	onclick="showAddWorkspaceDialog()"
	title="导入目录"
	>
	<button
	class="import-dir-btn"
	onclick="showAddWorkspaceDialog()"
	title="导入目录"
	aria-label="导入目录"
	>