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/DataTable.tsx

@@ -0,0 +1,69 @@
+interface Column<T> {
+  key: string;
+  label: string;
+  render: (item: T) => React.ReactNode;
+  className?: string;
+}
+
+interface DataTableProps<T> {
+  columns: Column<T>[];
+  data: T[];
+  onRowClick?: (item: T) => void;
+  emptyMessage?: string;
+  isLoading?: boolean;
+}
+
+export default function DataTable<T>({ columns, data, onRowClick, emptyMessage, isLoading }: DataTableProps<T>) {
+  if (isLoading) {
+    return (
+      <div className="flex items-center justify-center py-16 text-slate-400">
+        <svg className="animate-spin h-5 w-5 mr-3" viewBox="0 0 24 24">
+          <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4" fill="none" />
+          <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z" />
+        </svg>
+        Loading...
+      </div>
+    );
+  }
+
+  if (!data.length) {
+    return (
+      <div className="flex items-center justify-center py-16 text-slate-500">
+        {emptyMessage || 'No data found'}
+      </div>
+    );
+  }
+
+  return (
+    <div className="overflow-x-auto">
+      <table className="w-full text-sm">
+        <thead>
+          <tr className="border-b-2 border-slate-700">
+            {columns.map(col => (
+              <th key={col.key} className={`px-4 py-3 text-left text-xs font-semibold text-slate-400 uppercase tracking-wider ${col.className || ''}`}>
+                {col.label}
+              </th>
+            ))}
+          </tr>
+        </thead>
+        <tbody>
+          {data.map((item, i) => (
+            <tr
+              key={i}
+              onClick={() => onRowClick?.(item)}
+              className={`border-b border-slate-700/50 transition-colors hover:bg-blue-500/5 ${onRowClick ? 'cursor-pointer' : ''}`}
+            >
+              {columns.map(col => (
+                <td key={col.key} className={`px-4 py-3 ${col.className || ''}`}>
+                  {col.render(item)}
+                </td>
+              ))}
+            </tr>
+          ))}
+        </tbody>
+      </table>
+    </div>
+  );
+}
+
+export type { Column };