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/api/metrics.ts

@@ -0,0 +1,14 @@
+import client from "./client";
+import type { CoverageSummary, TacticCoverage } from "../types/models";
+
+/** Fetch the global coverage summary. */
+export async function getCoverageSummary(): Promise<CoverageSummary> {
+  const { data } = await client.get<CoverageSummary>("/metrics/summary");
+  return data;
+}
+
+/** Fetch coverage breakdown by tactic. */
+export async function getCoverageByTactic(): Promise<TacticCoverage[]> {
+  const { data } = await client.get<TacticCoverage[]>("/metrics/by-tactic");
+  return data;
+}