feat(m5): admin SPA pages for the template catalogue

- AdminTestsPage with filters (q, tactic, opsec, tag), modal-based CRUD,
  markdown textareas for procedure/result/detection, embedded MitreTagPicker
  for tagging.
- AdminScenariosPage with @dnd-kit/sortable drag-and-drop on the ordered
  test list, two-step save (PATCH metadata + PUT tests), catalogue picker
  excluding soft-deleted items.
- lib/templates.ts typed client + queryKey factory.
- MarkdownField helper (textarea with markdown hint label).
- Layout adds Tests + Scenarios admin nav links; App.tsx routes both
  behind RequireAdmin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -42,6 +42,8 @@ export function Layout() {
                     {navItem('/admin/users', 'Users')}
                     {navItem('/admin/groups', 'Groups')}
                     {navItem('/admin/invitations', 'Invitations')}
+                    {navItem('/admin/tests', 'Tests')}
+                    {navItem('/admin/scenarios', 'Scenarios')}
                   </>
                 )}
                 <span className="font-mono text-2xs text-text-dim ml-2" data-testid="me-email">
@@ -69,7 +71,7 @@ export function Layout() {
         <Outlet />
 
         <footer className="mt-[60px] py-8 border-t border-border text-center font-mono text-xs text-text-dim">
-          metamorph · M0 bootstrap · M1 db schema · M2 auth · M3 rbac · M4 mitre · design system from tasks/design.md
+          metamorph · M0 bootstrap · M1 db schema · M2 auth · M3 rbac · M4 mitre · M5 templates · design system from tasks/design.md
         </footer>
       </div>
     </div>