Apply stack design system: light theme + Lucide icons

- index.css: match portal token system (light body, dark login chrome)
- NoticeBoard: white cards/shadows, Lucide Pin/PinOff/Trash2/Plus/X icons,
  light input/button styles, category pills with navy active state
- AuthGate: explicit dark bg on login container (body is now light)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -9,6 +9,7 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "lucide-react": "^1.23.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1"
   },