Implement M5: hardening, input validation, and Vite+React+TS dashboard

Backend hardening:
- Fix 6 nginx.go non-constant format string build errors
- Add validation.go with hostname, PEM, and enum validators
- Apply input validation to all POST/PUT handlers (certificates,
  agents, CSR, policies, teams, owners, targets, issuers)
- Fix unchecked JSON decode in TriggerDeployment handler

Frontend (Vite + React + TypeScript):
- Migrate from single-file SPA to proper build pipeline
- 7 pages: Dashboard, Certificates (list+detail), Agents, Jobs,
  Notifications, Policies, Audit Trail
- TanStack Query for server state with auto-refetch intervals
- Certificate detail with version history and renewal trigger
- Job cancellation, status/type filtering, expiry countdowns
- Reusable components: DataTable, StatusBadge, ErrorState, PageHeader
- Dark theme with Tailwind CSS, sidebar nav via React Router

Server integration:
- Go server serves web/dist/ (Vite output) with SPA fallback
- Falls back to web/index.html for legacy mode
- .gitignore updated for web/node_modules/ and web/dist/

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

web/src/components/ErrorState.tsx

@@ -0,0 +1,21 @@
+interface ErrorStateProps {
+  error: Error;
+  onRetry?: () => void;
+}
+
+export default function ErrorState({ error, onRetry }: ErrorStateProps) {
+  return (
+    <div className="flex flex-col items-center justify-center py-16 text-slate-400">
+      <svg className="w-12 h-12 text-red-400 mb-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={1.5}>
+        <path strokeLinecap="round" strokeLinejoin="round" d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126zM12 15.75h.007v.008H12v-.008z" />
+      </svg>
+      <p className="text-sm mb-2">Failed to load data</p>
+      <p className="text-xs text-slate-500 mb-4">{error.message}</p>
+      {onRetry && (
+        <button onClick={onRetry} className="btn btn-primary text-xs">
+          Retry
+        </button>
+      )}
+    </div>
+  );
+}