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/src/components/AuthGate.tsx

@@ -75,9 +75,10 @@ export function AuthGate({ children }: Props) {
       <div style={{
         display: 'flex', flexDirection: 'column', alignItems: 'center',
         justifyContent: 'center', height: '100dvh', padding: '1.5rem',
+        background: 'var(--navy-dark)',
       }}>
         <div style={{
-          background: 'var(--surface)', borderRadius: 'var(--radius)',
+          background: 'var(--navy)', borderRadius: 'var(--radius)',
           padding: '2rem', width: '100%', maxWidth: '360px',
           border: '1px solid var(--surface-2)',
         }}>