refactor: import brand icons directly from design-system package

- Sidebar and sign-in logos use Vite import from @cameleer/design-system
- HTML favicons copied by postinstall script (gitignored)
- Remove manually copied PNGs from repo
- Clean up SecurityConfig permitAll (bundled assets under /_app/**)

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

ui/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",
-    "preview": "vite preview"
+    "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))\""
   },
   "dependencies": {
     "@cameleer/design-system": "0.1.33",