feat(dashboards): hover tooltips on all metric cards

New MetricTooltip component — a small ⓘ icon showing an executive-
friendly explanation panel on hover (CSS, no JS, instant).

DashboardPage: tooltips on all 6 coverage summary cards (Total
Techniques, Validated, Partial, In Progress, Not Covered, Not
Evaluated), Coverage Evolution chart, Test Pipeline funnel,
Team Activity and Validation Rate section headers.

ExecutiveDashboardPage: tooltips on all 4 sub-scores (Coverage,
Detection, Critical, Response), Score Trend, Top Threat Actors,
4 KPIs (MTTD, MTTR, Detection Efficacy, Validation Throughput),
Coverage by Tactic, Critical Gaps table, and all 6 team metrics
(Red/Blue Tests Done, Avg Time, Rejection).

Each tooltip explains what the metric measures, what a good/bad
value looks like, and what action to take — written for non-
technical executives.

frontend/src/components/CoverageSummaryCard.tsx

@@ -1,4 +1,5 @@
 import type { ReactNode } from "react";
+import MetricTooltip from "./MetricTooltip";
 
 interface CoverageSummaryCardProps {
   title: string;
@@ -7,6 +8,8 @@ interface CoverageSummaryCardProps {
   icon: ReactNode;
   colorClass: string;
   bgClass: string;
+  /** Optional tooltip explaining this metric to non-technical users */
+  tooltip?: { description: string; context?: string };
 }
 
 export default function CoverageSummaryCard({
@@ -16,6 +19,7 @@ export default function CoverageSummaryCard({
   icon,
   colorClass,
   bgClass,
+  tooltip,
 }: CoverageSummaryCardProps) {
   const percentage = total && total > 0 ? ((value / total) * 100).toFixed(1) : null;
 
@@ -23,7 +27,16 @@ export default function CoverageSummaryCard({
     <div className={`rounded-xl border border-gray-800 ${bgClass} p-5`}>
       <div className="flex items-center justify-between">
         <div>
-          <p className="text-sm font-medium text-gray-400">{title}</p>
+          <p className="text-sm font-medium text-gray-400 flex items-center">
+            {title}
+            {tooltip && (
+              <MetricTooltip
+                title={title}
+                description={tooltip.description}
+                context={tooltip.context}
+              />
+            )}
+          </p>
           <p className={`mt-1 text-3xl font-bold ${colorClass}`}>{value}</p>
           {percentage !== null && (
             <p className="mt-1 text-xs text-gray-500">{percentage}% of total</p>