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/views/AppLayout.vue

@@ -185,8 +185,14 @@ const defaultNav = [
 ]
 
 function buildNavItems(items) {
+  // Drop items the user cannot reach: a nav item may carry a `permission`
+  // (e.g. Slides -> slides.manage); hide it from anyone who lacks it so the
+  // link does not dead-end at the router guard. Admins hold every permission.
+  const visible = items.filter(item =>
+    !item.permission || authStore.hasPermission(item.permission))
+
   // Sort by position
-  const sorted = [...items].sort((a, b) => (a.position || 99) - (b.position || 99))
+  const sorted = [...visible].sort((a, b) => (a.position || 99) - (b.position || 99))
 
   // Assign section headers based on position ranges
   const result = []