feat: Phase 8 - Frontend main views (T-026 to T-031)

Implement all main frontend views for the MITRE ATT&CK coverage platform:

- T-026: Dashboard with coverage summary cards and tactic breakdown table

- T-027: Interactive ATT&CK matrix with filtering by status, tactic, platform

- T-028: Technique detail page with tests, intel items, and review actions

- T-029: Test creation form with technique selector and validation

- T-030: Test detail page with drag and drop evidence upload and download

- T-031: System admin panel with MITRE sync and intel scan controls

New components: CoverageSummaryCard, TacticCoverageChart, AttackMatrix, TechniqueCell, TestForm, EvidenceUpload, EvidenceList

New API modules: metrics.ts, techniques.ts, tests.ts, evidence.ts, system.ts

All views use TanStack Query for data fetching with proper loading and error states. Role-based UI controls for admin/lead actions.

frontend/src/components/CoverageSummaryCard.tsx

@@ -0,0 +1,38 @@
+import type { ReactNode } from "react";
+
+interface CoverageSummaryCardProps {
+  title: string;
+  value: number;
+  total?: number;
+  icon: ReactNode;
+  colorClass: string;
+  bgClass: string;
+}
+
+export default function CoverageSummaryCard({
+  title,
+  value,
+  total,
+  icon,
+  colorClass,
+  bgClass,
+}: CoverageSummaryCardProps) {
+  const percentage = total && total > 0 ? ((value / total) * 100).toFixed(1) : null;
+
+  return (
+    <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={`mt-1 text-3xl font-bold ${colorClass}`}>{value}</p>
+          {percentage !== null && (
+            <p className="mt-1 text-xs text-gray-500">{percentage}% of total</p>
+          )}
+        </div>
+        <div className={`rounded-lg p-3 ${bgClass} border border-gray-700`}>
+          {icon}
+        </div>
+      </div>
+    </div>
+  );
+}