feat: add social sharing identity

frontend/public/manifest.webmanifest

@@ -1,7 +1,7 @@
 {
   "name": "续想",
   "short_name": "续想",
-  "description": "让零散念头继续生长的私人笔记本",
+  "description": "一个先保存、后理解的私人 AI 笔记本。自由记录零散念头，由 AI 在后台整理并跟踪它们如何生长。",
   "start_url": "/",
   "display": "standalone",
   "background_color": "#f3f0e9",
@@ -12,7 +12,12 @@
       "sizes": "any",
       "type": "image/svg+xml",
       "purpose": "any maskable"
+    },
+    {
+      "src": "/logo-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
     }
   ]
 }
-