slides: gate management on slides.manage permission (grantable to non-admin curator)

The lobby-display and screensaver slide manager was admin-only. Add a shared
slides.manage permission so a curator can manage both surfaces without full
admin. Admins keep access via the require_permission admin bypass.

Backend:
- plugins/slides/api/routes.py: all 5 management routes require slides.manage
- plugins/slides/plugin.py: declare it via get_permissions(); nav item carries
  the permission so the frontend can gate visibility
- shopdb/core/api/auth.py: login response now returns the user's permissions
  (matches /me) so the frontend authStore has them on fresh login

Frontend:
- stores/auth.js: hasPermission(name) getter (admin true, else granted list)
- router/index.js: guard supports requiresPermission
- views/AppLayout.vue: hide nav items whose permission the user lacks
- plugins/slides/frontend/routes.js: slide manager gated requiresPermission

Tests: no-perm user 403, curator role with the perm 200 (+ login advertises
it), admin 200 via bypass.

Deploy: run `flask seed permissions` to create the row, then grant it to a
role in Settings > Users & Roles.

frontend/src/router/index.js

@@ -167,6 +167,11 @@ router.beforeEach(async (to, from, next) => {
   if (to.meta.requiresAdmin && !authStore.isAdmin) {
     return next('/')
   }
+  // A route may gate on a named permission instead of full admin (e.g. the
+  // slide manager -> slides.manage). Admins pass via hasPermission.
+  if (to.meta.requiresPermission && !authStore.hasPermission(to.meta.requiresPermission)) {
+    return next('/')
+  }
   if (to.meta.guest && authStore.isAuthenticated) {
     return next('/')
   }