feat(Pre-2.1.0-E): GUI completeness — 5 new pages, clickable nav, verification badges

Wire all remaining backend features to the frontend GUI:

New pages:
- DigestPage: preview digest HTML via iframe + send with confirmation
- ObservabilityPage: health status, metrics gauges, Prometheus config + live output
- JobDetailPage: full job details, verification section, timeline, audit events
- IssuerDetailPage: redacted config, test connection, issued certificates list
- TargetDetailPage: config, agent link, deployment history with verification

Existing page updates:
- JobsPage: clickable job IDs, verification column with VerificationBadge
- IssuersPage: clickable issuer names linking to detail page
- TargetsPage: clickable target names linking to detail page
- Sidebar: Digest and Observability nav items
- 5 new routes in main.tsx

API client: getJob, getIssuer, getTarget, getJobVerification, getPrometheusMetrics
Tests: 7 new Vitest tests (203 total), testing-guide Part 37 (17 manual tests)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/pages/TargetsPage.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react';
+import { Link } from 'react-router-dom';
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 import { getTargets, createTarget, deleteTarget } from '../api/client';
 import PageHeader from '../components/PageHeader';
@@ -266,7 +267,9 @@ export default function TargetsPage() {
       label: 'Target',
       render: (t) => (
         <div>
-          <div className="font-medium text-ink">{t.name}</div>
+          <Link to={`/targets/${t.id}`} className="font-medium text-accent hover:text-accent-bright" onClick={(e) => e.stopPropagation()}>
+            {t.name}
+          </Link>
           <div className="text-xs text-ink-faint font-mono">{t.id}</div>
         </div>
       ),