feat: Introduce a shared Layout component, add new Insights and Sources pages, and refactor Dashboard and Profile pages to integrate with the new layout.

frontend/src/pages/Insights.tsx

@@ -0,0 +1,18 @@
+export function InsightsPage() {
+    return (
+        <div className="page">
+            <header className="page-header">
+                <h1>Insights</h1>
+                <p className="text-secondary">AI-powered analysis of your health data</p>
+            </header>
+
+            <div className="card" style={{ textAlign: 'center', padding: 'var(--space-xl)' }}>
+                <div style={{ fontSize: '48px', marginBottom: 'var(--space-md)' }}>🚀</div>
+                <h3>Coming Soon</h3>
+                <p className="text-secondary">
+                    AI-generated health insights and recommendations based on your biomarker data.
+                </p>
+            </div>
+        </div>
+    )
+}