feat(desktop): 添加悬浮球 Quick Ask 功能

- 实现浅灰色玻璃拟态悬浮球，带机器人头图标
- 支持点击展开对话框，淡入淡出动画
- 支持窗口拖拽，区分点击和拖拽操作
- macOS 透明窗口支持 (macOSPrivateApi)
- 悬浮球 hover 放大效果，不溢出窗口
- 添加系统托盘 Toggle Quick Ask 菜单

packages/desktop/src-tauri/capabilities/default.json

@@ -5,6 +5,15 @@
   "windows": ["*"],
   "permissions": [
     "core:default",
+    "core:window:allow-show",
+    "core:window:allow-hide",
+    "core:window:allow-close",
+    "core:window:allow-set-focus",
+    "core:window:allow-set-position",
+    "core:window:allow-set-size",
+    "core:window:allow-start-dragging",
+    "core:window:allow-is-visible",
+    "core:webview:allow-create-webview-window",
     "shell:default",
     "fs:default",
     "dialog:default",