fix: skip postinstall favicon copy when public/ absent (Docker build)

The Dockerfile copies package.json before ui/ contents, so public/
doesn't exist during npm ci. Skip the copy gracefully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/package.json

@@ -7,7 +7,7 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "preview": "vite preview",
-    "postinstall": "node -e \"const fs=require('fs'),p='node_modules/@cameleer/design-system/assets/';[['cameleer3-32.png','public/favicon-32.png'],['cameleer3-192.png','public/favicon-192.png']].forEach(([s,d])=>fs.copyFileSync(p+s,d))\""
+    "postinstall": "node -e \"const fs=require('fs'),p='node_modules/@cameleer/design-system/assets/';if(fs.existsSync('public')){[['cameleer3-32.png','public/favicon-32.png'],['cameleer3-192.png','public/favicon-192.png']].forEach(([s,d])=>fs.copyFileSync(p+s,d))}\""
   },
   "dependencies": {
     "@cameleer/design-system": "0.1.34",