feat: 为 macOS 适配无边框窗口圆角效果

新增 macOS 平台配置文件，修改平台检测逻辑，在 macOS 时给 html 加标记，调整 CSS 让 #app 绘制背景并裁切 8px 圆角，避免首帧方角闪烁，同时适配弹窗遮罩圆角。

src/composables/usePlatform.ts

@@ -5,15 +5,17 @@ export type Platform = "windows" | "macos" | "linux" | "unknown"
  * 判断当前运行的操作系统类型。
  * 通过 navigator.userAgent 推断，避免额外引入 @tauri-apps/plugin-os。
  */
-export function usePlatform() {
+export function detectPlatform(): Platform {
   const ua = (navigator.userAgent || "").toLowerCase()
+  if (ua.includes("mac") || ua.includes("darwin")) return "macos"
+  if (ua.includes("win")) return "windows"
+  if (ua.includes("linux") || ua.includes("x11")) return "linux"
+  return "unknown"
+}
 
-  const platform = computed<Platform>(() => {
-    if (ua.includes("mac") || ua.includes("darwin")) return "macos"
-    if (ua.includes("win")) return "windows"
-    if (ua.includes("linux") || ua.includes("x11")) return "linux"
-    return "unknown"
-  })
+/** 在组件中响应式地获取当前平台 */
+export function usePlatform() {
+  const platform = ref<Platform>(detectPlatform())
 
   const isMac = computed(() => platform.value === "macos")
   const isWindows = computed(() => platform.value === "windows")