fix(desktop): blend macOS titlebar into sidebar

[Luhaozhu]

Summary / 概述

Follow-up to #21 after the original PR was merged.

The macOS traffic-light safe area used a flat #F5F6F7 background while the sidebar below it renders a translucent blue gradient over the same base. This left a visible gray block above the sidebar.

This change reuses the sidebar's exact first gradient layer in the safe area and keeps the existing dynamic sidebar-width tracking, so the titlebar remains visually continuous in both expanded and collapsed sidebar states. Layout and interaction behavior are unchanged.

Related issue / 关联 Issue

Follow-up to #21.

Type of change / 变更类型

• 📖 Documentation (document/**, README*.md)
• 🧩 Example / sample (examples/**)
• 🔧 Repo meta (CI, templates, .github/**)
• Other (please describe / 请说明): Maintainer-generated desktop shell fix synchronized from the upstream source repository.

Checklist / 检查项

• My change only touches non-generated paths (docs / examples / repo meta). 仅改动非生成路径。
• Links and code snippets were verified. 链接与代码片段已验证。
• For bilingual docs, I updated both document/en/** and document/zh-CN/** where applicable. 双语文档已同步更新（如适用）。
• I read CONTRIBUTING.md. 我已阅读贡献指南。

Validation / 验证

• cargo fmt --all -- --check
• cargo test proxy::tests --lib — 7 passed
• cargo test --lib — 20 passed
• git diff --check
• HUGAGENT_RELEASE_BUILD=1 python scripts/build_ce.py --import-check --pytest-check --frontend-check

The repository root has no package.json, so the generic npm run preflight command is not available; the repository's formal CE release checks above completed successfully, including the frontend production build.