icon-512-maskable.png — 新增maskable专用图标，logo缩小到70%并居中，留出安全区域，Android自适应图标不会裁切

manifest.json 修改：
purpose 拆分为两个图标："any" (普通) 和 "maskable" (Android自适应)
background_color 从黑色 #161617 改为白色 #ffffff，启动画面不会黑底
apple-touch-icon.png — 之前已修复为白色实底背景，iOS不再有黑边

public/manifest.json

@@ -4,14 +4,20 @@
   "description": "Vitiens - Task Platform",
   "start_url": "/",
   "display": "standalone",
-  "background_color": "#161617",
+  "background_color": "#ffffff",
   "theme_color": "#E3AC00",
   "icons": [
     {
       "src": "/icon-512.png",
       "sizes": "512x512",
       "type": "image/png",
-      "purpose": "any maskable"
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512-maskable.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     },
     {
       "src": "/logo.svg",