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/i18n/locales/nl/translation.json

@@ -177,7 +177,10 @@
                 "Available": "Beschikbaar",
                 "Maintenance": "Onderhoud",
                 "NotAvailable": "Niet beschikbaar"
-            }
+            },
+            "masterControlledTitle": "Beheerd door Master-CMS",
+            "masterControlled": "Deze instantie is door de Master-CMS uitgeschakeld. De beschikbaarheid kan hier niet worden gewijzigd totdat de Master de status weer vrijgeeft.",
+            "masterControlledSaveError": "Kan niet worden gewijzigd: de Master-CMS beheert deze status."
         },
         "modules": { "title": "Modulebeheer", "comingSoon": "Binnenkort beschikbaar" },
         "systemConfig": { "title": "Systeemconfiguratie", "comingSoon": "Binnenkort beschikbaar" },
@@ -243,6 +246,8 @@
         "generic": "Er is iets misgegaan. Probeer het opnieuw.",
         "accessDenied": "Je hebt geen toestemming om deze pagina te bekijken.",
         "setupRequired": "Systeeminstallatie vereist. Initialiseer eerst het systeem.",
-        "invalidInvitationToken": "Ongeldig of verlopen uitnodigingstoken."
+        "invalidInvitationToken": "Ongeldig of verlopen uitnodigingstoken.",
+        "featureUnavailableTitle": "Niet beschikbaar op deze instantie",
+        "featureUnavailable": "Deze functie is alleen beschikbaar op een Master-CMS-instantie."
     }
 }