🔧 fix(appearance): 修复透明通透失效并统一 Win/Mac 视觉强度

- 新增 macOS 原生窗口通透补强与启动重试，修复偶发不生效
- 引入跨平台透明/模糊映射，统一 Win/Mac 同滑块值观感
- 调整主窗口圆角与裁剪，优化整体视觉一致性

frontend/src/utils/appearance.ts

@@ -0,0 +1,62 @@
+const DEFAULT_OPACITY = 0.95;
+const MIN_OPACITY = 0.1;
+const MAX_OPACITY = 1.0;
+
+// macOS 端进一步增强通透感：同滑块值下更低等效不透明度、降低过重模糊。
+const MAC_OPACITY_FACTOR = 0.20;
+const MAC_BLUR_FACTOR = 1.00;
+const WINDOWS_OPACITY_FACTOR = 0.20;
+const WINDOWS_BLUR_FACTOR = 1.00;
+
+const clamp = (value: number, min: number, max: number) => Math.min(max, Math.max(min, value));
+
+export const isMacLikePlatform = (): boolean => {
+  if (typeof navigator === 'undefined') {
+    return false;
+  }
+  const platform = navigator.platform || '';
+  const ua = navigator.userAgent || '';
+  return /(Mac|iPhone|iPad|iPod)/i.test(`${platform} ${ua}`);
+};
+
+export const isWindowsPlatform = (): boolean => {
+  if (typeof navigator === 'undefined') {
+    return false;
+  }
+  const platform = navigator.platform || '';
+  const ua = navigator.userAgent || '';
+  return /(Win|Windows)/i.test(`${platform} ${ua}`);
+};
+
+const getPlatformFactors = () => {
+  if (isMacLikePlatform()) {
+    return { opacity: MAC_OPACITY_FACTOR, blur: MAC_BLUR_FACTOR };
+  }
+  if (isWindowsPlatform()) {
+    return { opacity: WINDOWS_OPACITY_FACTOR, blur: WINDOWS_BLUR_FACTOR };
+  }
+  return undefined;
+};
+
+export const normalizeOpacityForPlatform = (opacity: number | undefined): number => {
+  const raw = clamp(opacity ?? DEFAULT_OPACITY, MIN_OPACITY, MAX_OPACITY);
+  const factors = getPlatformFactors();
+  if (!factors) {
+    return raw;
+  }
+
+  return clamp(MIN_OPACITY + (raw - MIN_OPACITY) * factors.opacity, MIN_OPACITY, MAX_OPACITY);
+};
+
+export const normalizeBlurForPlatform = (blur: number | undefined): number => {
+  const raw = Math.max(0, blur ?? 0);
+  const factors = getPlatformFactors();
+  if (!factors) {
+    return raw;
+  }
+  return Math.round(raw * factors.blur);
+};
+
+export const blurToFilter = (blur: number): string | undefined => {
+  return blur > 0 ? `blur(${blur}px)` : undefined;
+};