Completes local-dev-master-slave-setup: dual-instance frontend tooling, module-capability gating, and master/slave protocol self-healing fixes

Frontend (Unit 2 completion): dual dev-server tooling (pnpm dev:slave,
pnpm dev:all), per-instance browser tab titles, and a backend
capability check (SystemController + useSystemCapabilities +
ModuleGuard) so a Master-only page is hidden on a slave instance
instead of assuming every backend has every module.

Master/slave protocol fixes surfaced by actually running master and
slave side by side locally:
- Deactivating a CMS instance (Inactive) now releases the slave's
  master gate instead of leaving it stuck on its last pushed status.
- The periodic integrity check now also re-pushes status to every
  reachable slave (previously URL-verification only) and runs once
  immediately on startup.
- Added the originally-specified (but never implemented) slave-pull
  path: a slave now periodically polls its own status from the master
  (GET /api/v1/SlaveStatus) and fails open to Available if the master
  is unreachable for too long, complementing the existing push.
- The slave's own Settings page can no longer "successfully" change
  local availability while the master controls it; it's now locked
  with an explanatory banner and the backend rejects the write with
  409 instead of silently no-op'ing it.
- CMS instance status badges now match the dashboard's color/icon
  styling instead of a plain grey badge.

Also corrected the master-cms-module design docs to match this
as-built behavior, and flagged (without a full rewrite) a larger,
pre-existing divergence between its inception-stage application
design and what construction actually built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -5,6 +5,7 @@ import type { LucideIcon } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import { useAuth } from '@/contexts/auth-context';
 import { useAvailabilityStatus } from '@/api/useAvailability';
+import { useSystemCapabilities } from '@/api/useSystemCapabilities';
 import { LanguageSwitcher } from '@/i18n/LanguageSwitcher';
 import { ThemeToggle } from './ThemeToggle';
 import { UserMenu } from './UserMenu';
@@ -17,6 +18,7 @@ interface NavItem {
     icon: LucideIcon;
     testId: string;
     roles?: Role[];
+    requiredModule?: string;
 }
 
 const ALLOWED_WHEN_UNAVAILABLE = ['/dashboard', '/settings'];
@@ -24,7 +26,7 @@ const ALLOWED_WHEN_UNAVAILABLE = ['/dashboard', '/settings'];
 const NAV_ITEMS: NavItem[] = [
     { to: '/dashboard', labelKey: 'nav.dashboard', icon: LayoutDashboard, testId: 'nav-dashboard' },
     { to: '/users', labelKey: 'nav.users', icon: Users, testId: 'nav-users', roles: ['Owner', 'Administrator'] },
-    { to: '/cms', labelKey: 'nav.cms', icon: FileText, testId: 'nav-cms', roles: ['Owner'] },
+    { to: '/cms', labelKey: 'nav.cms', icon: FileText, testId: 'nav-cms', roles: ['Owner'], requiredModule: 'Master' },
 ];
 
 const SETTINGS_ITEM: NavItem = {
@@ -39,11 +41,14 @@ export function Sidebar({ onClose }: SidebarProps = {}) {
     const { t } = useTranslation();
     const { user } = useAuth();
     const { data: availability } = useAvailabilityStatus();
+    const { data: capabilities } = useSystemCapabilities();
 
     const role = user?.role as Role | undefined;
     const systemUnavailable = availability?.status === 'NotAvailable';
     const visibleItems = NAV_ITEMS.filter(
-        (item) => !item.roles || (role && item.roles.includes(role))
+        (item) =>
+            (!item.roles || (role && item.roles.includes(role))) &&
+            (!item.requiredModule || capabilities?.modules.includes(item.requiredModule))
     );
     const showSettings = !SETTINGS_ITEM.roles || (role && SETTINGS_ITEM.roles.includes(role));