Add hamburger nav menu with 4-page navigation structure

- NavDrawer component: slide-in left drawer with backdrop, matches dark theme
- Nav items: Home, Reporting, Knowledge Base, Exports with color-coded icons
- Active page highlighted with colored background + indicator dot
- Placeholder pages for Reporting (amber), Knowledge Base (green), Exports (purple)
- Stats bar and three-column layout conditionally render on Home page only
- currentPage state drives all page switching

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

frontend/src/components/pages/KnowledgeBasePage.js

@@ -0,0 +1,25 @@
+import React from 'react';
+import { BookOpen } from 'lucide-react';
+
+export default function KnowledgeBasePage() {
+  return (
+    <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'center', minHeight: '60vh' }}>
+      <div style={{ textAlign: 'center' }}>
+        <div style={{
+          width: '72px', height: '72px', borderRadius: '1rem', margin: '0 auto 1.5rem',
+          background: 'rgba(16, 185, 129, 0.1)',
+          border: '1px solid rgba(16, 185, 129, 0.3)',
+          display: 'flex', alignItems: 'center', justifyContent: 'center'
+        }}>
+          <BookOpen style={{ width: '36px', height: '36px', color: '#10B981' }} />
+        </div>
+        <h2 style={{ fontFamily: 'monospace', fontSize: '1.5rem', fontWeight: '700', color: '#10B981', textTransform: 'uppercase', letterSpacing: '0.1em', marginBottom: '0.5rem' }}>
+          Knowledge Base
+        </h2>
+        <p style={{ color: '#475569', fontSize: '0.875rem', fontFamily: 'monospace' }}>
+          Under construction — coming soon
+        </p>
+      </div>
+    </div>
+  );
+}