fix(dsh): stop long-thought scroll layout oscillation

[starsstreaming]

DSH 的渐隐类附着在原生滚动内容 body 上。背景插件将所有 _fade 类元素设为 display:none，使 ResizeObserver 反复观察到可见高度和零高度；已完成长思考展开后也会持续抖动。

删除普通背景和画窗中的两处隐藏规则，保留宿主内容布局及渐隐 mask。新增浏览器回归验证两种背景模式下的高度、滚动位置与 mask，并删除要求错误隐藏行为的旧断言。

Closes #87

验证：实际脚本的 Edge 浏览器与插件测试共 22 项通过，无失败或跳过。Desktop 0.2.0-rc.2 中同一已完成长思考在修复后连续 180 帧稳定为 400px；恢复旧规则会再次出现隐藏/显示循环。未执行新请求流式或长期压力测试。

依赖 PR #88 的测试夹具；本 PR 只提交滚动布局修复，正式 npm 1.0.29 已包含此修复，发布证据另行提交。

GitHub CI 已完成，但未全绿：Linux 安装 Windows 专用 workspace 报 EBADPLATFORM；Windows 的快捷方式现场测试失败。这些故障同样出现在未改动的 main 基线 CI run 37138973708，本次未改动对应代码。最终源码树本地完整测试 397 项通过、1 项既有跳过，类型检查通过；独立审查无阻塞项。

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve suppression for the conversation-list fade

When an ordinary or gallery background is active, removing this rule also restores DSH's decorative conversation-list bottom fade, which docs/deepseek-harness.md:87 explicitly says is disabled because it renders a dark band over the wallpaper. The repository-wide search finds no replacement suppression, so fixing the reasoning-body oscillation by deleting the broad selector regresses every wallpaper view containing that list; retain a narrower rule targeting the decorative fade while excluding the [data-step-process-body] scroll content.

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