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/.env.example

@@ -4,3 +4,16 @@
 # Set CookieSameSite=None in appsettings.Development.json so the cookie
 # is sent cross-origin when the frontend and backend run on different ports.
 VITE_API_BASE_URL=https://localhost:7221
+
+# Browser tab title. Defaults to "SlpModularCms" if unset — override so the
+# tab is instantly recognizable when running multiple instances side by side
+# (e.g. local master vs. slave, see below).
+VITE_APP_TITLE=SlpModularCms
+
+# --- Local master/slave dev setup ---
+# To point the frontend at the slave instance (SlpModularCms.Api.Slave,
+# see root README.md "Lokaal Master + Slave Draaien (Dev)") instead of the
+# master, copy this file to `.env.slave.local` with:
+#   VITE_API_BASE_URL=https://localhost:7222
+#   VITE_APP_TITLE=SlpModularCms (Slave)
+# and run `pnpm dev:slave` (uses --mode slave, port 5174) instead of `pnpm dev`.