feat: macOS 改用原生圆角与原生红绿灯

- tauri.conf.json: decorations=true + titleBarStyle=Overlay + transparent
  + hiddenTitle + trafficLightPosition，启用原生圆角/squircle 与原生红绿灯
- main.css: 移除 #root 的 CSS border-radius，避免圆弧覆盖原生连续曲率
- App.tsx: macOS 下隐藏自绘 Windows 风格三按钮
- ContentArea.tsx: 侧栏隐藏（沉浸/竖屏）时 header 左侧为红绿灯留白
- Sidebar.tsx: macOS 下 logo 区上内边距增大，避开红绿灯

Co-Authored-By: Claude <noreply@anthropic.com>

src-tauri/tauri.conf.json

@@ -19,8 +19,11 @@
         "height": 680,
         "resizable": true,
         "fullscreen": false,
-        "decorations": false,
-        "transparent": false,
+        "decorations": true,
+        "transparent": true,
+        "hiddenTitle": true,
+        "titleBarStyle": "Overlay",
+        "trafficLightPosition": { "x": 16, "y": 22 },
         "center": true,
         "minWidth": 360,
         "minHeight": 640