feat(chat): add local-first conversation workspace

chat/public/manifest.webmanifest

@@ -1,18 +1,20 @@
 {
+  "id": "/chat/",
   "name": "Xiteng Chat",
   "short_name": "Chat",
-  "description": "使用个人 Key Vault 凭据的轻量 AI 对话界面",
-  "start_url": "/",
-  "scope": "/",
+  "description": "本地优先、可选登录同步的轻量 AI 对话界面",
+  "start_url": "/chat/",
+  "scope": "/chat/",
   "display": "standalone",
+  "launch_handler": {"client_mode": "navigate-existing"},
   "background_color": "#f4f4f0",
   "theme_color": "#171717",
   "orientation": "any",
   "lang": "zh-CN",
   "categories": ["productivity", "utilities"],
   "icons": [
-    {"src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png"},
-    {"src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png"},
+    {"src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any"},
+    {"src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any"},
     {"src": "/icons/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable"}
   ]
 }