feat: add tooltip to Overall Score gauge in Executive Dashboard

frontend/src/pages/ExecutiveDashboardPage.tsx

@@ -76,7 +76,15 @@ function ScoreGauge({ score, label }: { score: number; label: string }) {
           <span className="text-[10px] text-gray-500">/ 100</span>
         </div>
       </div>
-      <span className="mt-2 text-xs font-medium text-gray-400">{label}</span>
+      <div className="mt-2 flex items-center gap-1">
+        <span className="text-xs font-medium text-gray-400">{label}</span>
+        <MetricTooltip
+          title="Overall Security Score"
+          description="A composite score (0–100) representing the organisation's overall security posture. Calculated from 4 dimensions: Coverage, Detection maturity, Critical technique coverage, and Response readiness."
+          context="< 30 = Critical gaps  |  30–50 = High risk  |  50–70 = Improving  |  > 70 = Good posture"
+          position="below"
+        />
+      </div>
     </div>
   );
 }