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/api/types.ts

@@ -115,4 +115,13 @@ export interface AvailabilityResponse {
     status: AvailabilityStatus;
     checkedAt: string; // ISO 8601
     message: string;
+    /** True when a Master CMS has taken control of this instance's gate — the local status cannot be changed here. */
+    isMasterControlled: boolean;
+}
+
+// Which optional modules (e.g. "Master") this backend instance has loaded —
+// lets the frontend tell a master-only feature apart from a slave instance
+// without that module (local master/slave dev setup).
+export interface SystemCapabilities {
+    modules: string[];
 }

frontend/src/api/useSystemCapabilities.ts

@@ -0,0 +1,19 @@
+import { useQuery } from '@tanstack/react-query';
+import { api } from '@/lib/api-client';
+import type { SystemCapabilities } from './types';
+
+// Backend may return PascalCase (Modules) if no camelCase policy is set.
+type RawSystemCapabilities = { modules?: string[]; Modules?: string[] };
+
+export function useSystemCapabilities() {
+    return useQuery<SystemCapabilities, Error>({
+        queryKey: ['system', 'capabilities'],
+        queryFn: async () => {
+            const raw = await api.get<RawSystemCapabilities>('/api/v1/System/capabilities');
+            return { modules: raw.modules ?? raw.Modules ?? [] };
+        },
+        // Which modules a backend has loaded is fixed for the lifetime of that
+        // backend process — no need to ever refetch within a session.
+        staleTime: Infinity,
+    });
+}