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.

plugins/slides/plugin.py

@@ -58,14 +58,25 @@ class SlidesPlugin(BasePlugin):
         """Slide playlist metadata (image files live on disk)."""
         return [TvSlide]
 
+    def get_permissions(self) -> List:
+        """RBAC permission for curating slides (both surfaces). Grantable to a
+        non-admin role so a curator can manage the lobby TV + screensaver
+        without full admin. Admins hold it by default."""
+        return [
+            ('slides.manage',
+             'Manage lobby display and screensaver slides', 'slides'),
+        ]
+
     def get_navigation_items(self) -> List[Dict]:
-        """Sidebar entry for the slide manager (admin)."""
+        """Sidebar entry for the slide manager. Gated on slides.manage so only a
+        curator (or admin, who holds it by default) sees the link."""
         return [
             {
                 'name': 'Slides',
                 'icon': 'image',
                 'route': '/settings/slides',
                 'position': 7,
+                'permission': 'slides.manage',
             },
         ]