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>

aidlc-docs/features/local-dev-master-slave-setup/construction/plans/unit-2-frontend-dual-instance-tooling-code-generation-plan.md

@@ -10,14 +10,14 @@
 
 ## Steps
 
-- [ ] **Step 1 — Frontend env files for slave mode**
+- [x] **Step 1 — Frontend env files for slave mode**
   - Create `frontend/.env.slave.local` (gitignored via existing `frontend/.gitignore` `*.local` pattern — verified via `git check-ignore`): `VITE_API_BASE_URL=https://localhost:7222`
   - Modify `frontend/.env.example`: add a second documented block showing the slave-mode value, alongside the existing master-mode `VITE_API_BASE_URL` example
 
-- [ ] **Step 2 — `dev:slave` npm script**
+- [x] **Step 2 — `dev:slave` npm script**
   - Modify `frontend/package.json`: add `"dev:slave": "vite --mode slave --port 5174"` to the `scripts` section. Vite's mode-based env loading will load `.env.slave.local` when run with `--mode slave` (Vite loads `.env.[mode].local` in addition to `.env.local`; since both files would apply, and `.env.local` takes precedence per Vite's env-file priority for the same key when both exist for a mode, name the slave file `.env.slave.local` specifically — this file only loads when `--mode slave` is passed, so there is no conflict with the default `.env.local` used by `pnpm dev`)
 
-- [ ] **Step 3 — Runbook documentation**
+- [x] **Step 3 — Runbook documentation**
   - Modify root `README.md`: add new section **"Lokaal Master + Slave Draaien (Dev)"** immediately after the existing "Master CMS Module" section, covering:
     1. Starting the master backend (`dotnet run --project src/SlpModularCms.Api --launch-profile https`)
     2. Starting the slave backend (`dotnet run --project src/SlpModularCms.Api.Slave --launch-profile https`), noting it needs its own `appsettings.local.json` (from `appsettings.local.json.example`) with a separate local database
@@ -25,7 +25,7 @@
     4. Using the existing "Add CMS Instance" dialog on the master frontend to register the slave (URL `https://localhost:7222`) and confirm it shows as connected/healthy
     5. Cross-reference to this feature's requirements doc for anyone wanting the full rationale
 
-- [ ] **Step 4 — Documentation summary**
+- [x] **Step 4 — Documentation summary**
   - Create `aidlc-docs/features/local-dev-master-slave-setup/construction/unit-2-frontend-dual-instance-tooling/code/summary.md` documenting what was created/modified
 
 ## Notes