fix(desktop): 设置窗口尺寸回归内容实际大小 + 原生贴形阴影 + 红绿灯对齐 + 显式拖拽权限（12A 实测反馈）

- 去掉 16px CSS 阴影边距方案（窗口比观感大、外圈灰影、点击区域错位）：
  transparent 窗口开 macOS 原生阴影，阴影贴合内容圆角，窗口 440x560
  即卡片本体
- trafficLightPosition {16,12}：红绿灯垂直居中于 36px 自绘顶条，与
  「dudu 设置」标题对齐
- capabilities 显式加 core:window:allow-start-dragging，保证
  data-tauri-drag-region 拖拽可用

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

desktop/src-tauri/capabilities/default.json

@@ -2,6 +2,17 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "所有窗口的基础能力：核心 IPC（invoke/listen）+ 全局快捷键",
-  "windows": ["settings", "overlay", "tray", "login", "feedback", "onboarding"],
-  "permissions": ["core:default", "global-shortcut:default"]
+  "windows": [
+    "settings",
+    "overlay",
+    "tray",
+    "login",
+    "feedback",
+    "onboarding"
+  ],
+  "permissions": [
+    "core:default",
+    "global-shortcut:default",
+    "core:window:allow-start-dragging"
+  ]
 }