Free window header occluded by client top bar -> tab stuck at top / 浮窗页眉被顶栏遮挡，tab 卡在顶部拖不回来

[ldmuggle]

提交前确认 / Pre-submit checklist

• 我已搜索现有 issue，确认没有完全相同的条目（只有完全一致才算重复）| I searched existing issues and confirmed there is no exact duplicate (only exactly identical issues count as duplicates)
• 我已确认使用最新版本 dsh-better-sidebar / I confirmed I am on the latest dsh-better-sidebar version

运行环境 / Environment

Desktop 实现（如 deepseek-harness-desktop）/ Desktop build (e.g. deepseek-harness-desktop)

DSH Web 版本 + 浏览器 / DSH Web version + Browser

dsh-plugin-desktop v2.0.5

Desktop 实现仓库 / Desktop repo（源头 / upstream）

No response

插件版本 / Plugin version

v0.18.1（DSH Desktop 插件市场安装）

类别 / Category

🐛 Bug（功能异常 / Something is broken）

描述 / Description

现象 / What happened
浮窗（free window）模式下，把 tab 拖到客户端最顶部时，页眉（唯一拖拽把手）被顶栏遮挡，浮窗卡在顶部拖不回来。所有浮窗 tab 都受影响（不只某个插件）。
In free-window mode, dragging a tab to the very top hides the header (the only drag handle) under the client's top bar; the tab gets stuck and cannot be dragged out. Affects ALL floating tabs.

期望行为 / Expected behavior
浮窗页眉不被顶栏遮挡、随时能拖出；或浮窗 top 被钳制在不低于顶栏的位置。
The header should never be occluded by the top bar, so the window can always be dragged; or the float's top should be clamped below the top bar.

复现步骤 / Steps to reproduce (required)

1. 开任意 tab → 拖出成浮窗。 Open any tab → drag out to a floating window.
2. 把浮窗拖到客户端最顶部。 Drag the float window to the very top of the client.
3. 页眉被顶栏遮住，浮窗卡住、无法拖出。 The header is hidden under the top bar; the window is stuck.
4. （绕法）切到"无顶部"布局 → 拖出 → 切回。 Workaround: switch to the no-top layout, drag it out, switch back.

截图或日志 / Screenshots or logs
（可附一张"浮窗卡在顶部、页眉被遮挡"的截图）(attach a screenshot of the stuck window)

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客户端 / Client：DSH Desktop v2.0.5 (dsh-plugin-desktop)；默认布局有顶栏，另有"无顶部"布局模式（顶栏高度随布局模式变）。
根因 / Root cause：src/client/FreeWindow.tsx 的 clampMove 允许浮窗 top = 0（视口最顶），压在顶栏下；34px floatHeader 被遮挡。
建议修复 / Suggested fix：clampMove 的 top 下限改为 max(titlebarInset, panelHost.top)，自适应有顶/无顶布局：

const host = rootRef.current?.closest('[data-dsh-panel-host]')
const env = parseDesktopEnv()
const minTop = Math.max(env.titlebarInset, host ? host.getBoundingClientRect().top : 0)
// y 下限：Math.min(Math.max(y, minTop), Math.max(minTop, vh - float.h))

### 补充信息 / Additional context

_No response_