fix(dropdown): 展开层改 Portal + fixed 定位，逃出弹窗 overflow 裁剪

弹窗（.modal 带 overflow-y:auto）里的下拉展开层是 absolute 定位，
会被弹窗滚动盒裁掉下半截（Git 同步设置里实测被遮挡）。展开层改经
Portal 挂到 body、按触发钮 rect 用 fixed 定位：下方空间不足时
向上翻，maxHeight 按可用空间夹取；窗口 resize 时关闭。
useDismiss 泛化为可传多个 ref（触发器与 Portal 菜单两棵子树都算内）。
CSS 相应去掉 .dropdown-menu 的 absolute 定位（几何改由内联给出）。
验证：vitest 124+2（新增翻转几何断言：贴底触发钮翻上展开、
Portal 落点不在组件容器内）；无头 Chromium 实测弹窗内下拉菜单
完整落在视口内、8 项全显、已逃出弹窗 DOM。

tests/dropdown.test.tsx

@@ -22,18 +22,44 @@ describe('Dropdown', () => {
     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')
+    const menu = document.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()
+    expect(document.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()
+    expect(document.querySelector('.dropdown-menu')).not.toBeNull()
     await act(async () => { document.body.dispatchEvent(new MouseEvent('mousedown', { bubbles: true })) })
+    expect(document.querySelector('.dropdown-menu')).toBeNull()
+    await act(async () => { root.unmount() })
+    container.remove()
+  })
+
+  it('展开层经 Portal 挂到 body；触发钮靠近窗口底部时向上翻且不溢出', async () => {
+    const container = document.createElement('div')
+    document.body.appendChild(container)
+    const root = createRoot(container)
+    await act(async () => {
+      root.render(<Dropdown value="github" options={OPTIONS} onChange={() => undefined} />)
+    })
+    const button = container.querySelector('.dropdown-button') as HTMLButtonElement
+    // 模拟触发钮贴近窗口底部（jsdom 默认视口高 768）：下方只剩 30px
+    button.getBoundingClientRect = () => ({
+      top: 700, bottom: 730, left: 100, right: 300, width: 200, height: 30, x: 100, y: 700, toJSON: () => ({}),
+    }) as DOMRect
+    await act(async () => { button.dispatchEvent(new MouseEvent('click', { bubbles: true })) })
+    // Portal：菜单不在组件容器内，而是 body 直属
     expect(container.querySelector('.dropdown-menu')).toBeNull()
+    const menu = document.querySelector('.dropdown-menu') as HTMLElement | null
+    expect(menu).not.toBeNull()
+    expect(menu!.style.position).toBe('fixed')
+    // 3 项估算高 101，下方放不下 → 翻到触发钮上方：top = 700 - 101 - 4
+    expect(menu!.style.top).toBe('595px')
+    expect(menu!.style.left).toBe('100px')
+    expect(menu!.style.width).toBe('200px')
     await act(async () => { root.unmount() })
     container.remove()
   })