feat(frontend): sprint 5 — templates CRUD pages + nav + picker modal + dropdown

- types.ts: SimulationTemplate, SimulationTemplateCreateInput, SimulationTemplatePatchInput,
  extend SimulationCreateInput with template_id
- api/templates.ts: listTemplates, getTemplate, createTemplate, updateTemplate, deleteTemplate
- hooks/useTemplates.ts: useTemplates, useTemplate, useCreateTemplate, useUpdateTemplate,
  useDeleteTemplate (TanStack Query, invalidates ["templates"])
- TemplatesListPage: /admin/templates — table (name, MITRE count, created by, updated),
  New/Edit/Delete actions, loading/error/empty states
- TemplateFormPage: /admin/templates/new + /admin/templates/:id/edit — controlled form
  with inline MITRE field (picker + matrix modal), ConfirmDialog for delete
- TemplatePickerModal: reusable modal listing templates with empty state (AC-27.6)
- SimulationList: replace "New simulation" link with split-button dropdown
  (Blank → /simulations/new | From template… → TemplatePickerModal + POST template_id)
- Layout: "Templates" nav link (admin | redteam, before "Users")
- App.tsx: /admin/templates routes gated roles=["admin","redteam"]
- 26 new Vitest tests (118 total, 92 original preserved)

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

frontend/src/hooks/useTemplates.ts

@@ -0,0 +1,62 @@
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
+import {
+  createTemplate,
+  deleteTemplate,
+  getTemplate,
+  listTemplates,
+  updateTemplate,
+} from '@/api/templates';
+import type { SimulationTemplateCreateInput, SimulationTemplatePatchInput } from '@/api/types';
+
+function templatesKey() {
+  return ['templates'] as const;
+}
+
+function templateKey(id: number) {
+  return ['templates', id] as const;
+}
+
+export function useTemplates() {
+  return useQuery({
+    queryKey: templatesKey(),
+    queryFn: listTemplates,
+  });
+}
+
+export function useTemplate(id: number | undefined) {
+  return useQuery({
+    queryKey: id ? templateKey(id) : ['templates', 'none'],
+    queryFn: () => getTemplate(id as number),
+    enabled: typeof id === 'number' && !Number.isNaN(id),
+  });
+}
+
+export function useCreateTemplate() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (input: SimulationTemplateCreateInput) => createTemplate(input),
+    onSuccess: () => qc.invalidateQueries({ queryKey: templatesKey() }),
+  });
+}
+
+export function useUpdateTemplate(id: number) {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (patch: SimulationTemplatePatchInput) => updateTemplate(id, patch),
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: templateKey(id) });
+      qc.invalidateQueries({ queryKey: templatesKey() });
+    },
+  });
+}
+
+export function useDeleteTemplate() {
+  const qc = useQueryClient();
+  return useMutation({
+    mutationFn: (id: number) => deleteTemplate(id),
+    onSuccess: (_data, id) => {
+      qc.invalidateQueries({ queryKey: templateKey(id) });
+      qc.invalidateQueries({ queryKey: templatesKey() });
+    },
+  });
+}