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('/')
   }

frontend/src/stores/auth.js

@@ -13,6 +13,13 @@ export const useAuthStore = defineStore('auth', {
     roles: (state) => state.user?.roles || [],
     hasRole: (state) => (role) => state.user?.roles?.includes(role) || false,
     isAdmin: (state) => state.user?.roles?.includes('admin') || false,
+    // True if the user holds a named permission. Admins hold every permission
+    // (mirrors the backend require_permission admin bypass), so they pass
+    // regardless of the permissions list. Non-admins check their granted list.
+    hasPermission: (state) => (name) =>
+      state.user?.roles?.includes('admin')
+      || state.user?.permissions?.includes(name)
+      || false,
     // True when an admin-set temporary password must be changed before use.
     mustChangePassword: (state) => !!state.user?.mustchangepassword,
     // Full name from the employee directory (falls back to username/SSO).

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 = []