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/evidence.ts

@@ -0,0 +1,30 @@
+import client from "./client";
+
+export interface EvidenceOut {
+  id: string;
+  test_id: string;
+  file_name: string;
+  sha256_hash: string;
+  uploaded_by: string | null;
+  uploaded_at: string;
+  download_url: string;
+}
+
+/** Upload evidence file for a test. */
+export async function uploadEvidence(testId: string, file: File): Promise<EvidenceOut> {
+  const formData = new FormData();
+  formData.append("file", file);
+
+  const { data } = await client.post<EvidenceOut>(`/tests/${testId}/evidence`, formData, {
+    headers: {
+      "Content-Type": "multipart/form-data",
+    },
+  });
+  return data;
+}
+
+/** Get evidence metadata with download URL. */
+export async function getEvidence(evidenceId: string): Promise<EvidenceOut> {
+  const { data } = await client.get<EvidenceOut>(`/evidence/${evidenceId}`);
+  return data;
+}