feat(ui): 自绘 Dropdown 替代原生 select（外观设置·临时主题）

全应用唯一的原生 select 换为自绘下拉组件：按钮对齐 .input 样式，
展开层沿用应用菜单语言（选中 --accent-weak、悬停 --hover），复用
useDismiss 点别处/Esc 关闭，支持上下键选行。tests/dropdown 1 例。

tests/dropdown.test.tsx

@@ -0,0 +1,40 @@
+// 自绘下拉回归：展开/选中/点别处关闭（替代原生 select）。
+import { describe, it, expect, vi } from 'vitest'
+import { createRoot } from 'react-dom/client'
+import { act } from 'react-dom/test-utils'
+import { Dropdown } from '../src/components/Dropdown'
+
+const OPTIONS = [
+  { value: '', label: '跟随全局主题' },
+  { value: 'github', label: 'GitHub' },
+  { value: 'notion', label: 'Notion' },
+]
+
+describe('Dropdown', () => {
+  it('展开列出选项且标出选中项；点击选项回调并关闭；点别处关闭', async () => {
+    const container = document.createElement('div')
+    document.body.appendChild(container)
+    const root = createRoot(container)
+    const onChange = vi.fn()
+    await act(async () => {
+      root.render(<Dropdown value="github" options={OPTIONS} onChange={onChange} />)
+    })
+    const button = container.querySelector('.dropdown-button') as HTMLButtonElement
+    expect(button.textContent).toContain('GitHub')
+    await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
+    const menu = container.querySelector('.dropdown-menu')
+    expect(menu).not.toBeNull()
+    expect(menu?.querySelector('button.selected')?.textContent).toBe('GitHub')
+    const notion = [...menu!.querySelectorAll('button')].find((b) => b.textContent === 'Notion')
+    await act(async () => { notion?.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
+    expect(onChange).toHaveBeenCalledWith('notion')
+    expect(container.querySelector('.dropdown-menu')).toBeNull()
+    // 再次展开，点别处关闭
+    await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
+    expect(container.querySelector('.dropdown-menu')).not.toBeNull()
+    await act(async () => { document.body.dispatchEvent(new MouseEvent('mousedown', { bubbles: true })) })
+    expect(container.querySelector('.dropdown-menu')).toBeNull()
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+})