docs: remove admin SPA references and update production URLs

The admin SPA (apps/admin/) has been retired. Its functionality now
lives in apps/app/ under /platform/* routes for super_admin users.
Updated all documentation to reflect: 2 SPAs instead of 3, removed
FRONTEND_ADMIN_URL/port 5173 references, changed production URL from
app.crewli.app to crewli.app. Retired admin-specific security audit
findings (A13-2, A13-4, A13-5, A13-7) and APPS-01 backlog item.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev-docs/dev-guide.md

@@ -53,8 +53,7 @@ crewli/                                    # Monorepo root
 │   │   └── seeders/
 │   └── tests/Feature/Api/V1/             # PHPUnit feature tests per controller
 ├── apps/
-│   ├── admin/                             # Super Admin SPA (Vuexy)
-│   ├── app/                               # Organizer SPA (Vuexy) -- HOOFDAPP
+│   ├── app/                               # Organizer + Platform Admin SPA (Vuexy) -- HOOFDAPP
 │   └── portal/                            # Externe portals (vrijwilliger, artiest, leverancier)
 ├── docs/                                  # Design document, API docs, ERD
 │   ├── design-document.md
@@ -91,7 +90,7 @@ php artisan vendor:publish --provider="Spatie\Permission\PermissionServiceProvid
 php artisan vendor:publish --provider="Spatie\LaravelActivitylog\ActivitylogServiceProvider"
 ```
 
-**Frontend — alle apps (apps/app/, apps/admin/, apps/portal/)**
+**Frontend — alle apps (apps/app/, apps/portal/)**
 
 ```bash
 # TanStack Query voor API state management
@@ -103,7 +102,7 @@ npm install vee-validate zod @vee-validate/zod
 # Drag-and-drop (form builder, timetable, prioriteitsranking)
 npm install vuedraggable@next
 
-# In apps/app/ en apps/admin/ ook:
+# In apps/app/ ook:
 npm install @fullcalendar/vue3 @fullcalendar/timeline @fullcalendar/resource-timeline
 ```
 
@@ -138,8 +137,7 @@ Design Document: /resources/design/design-document.md
 
 ## Repository Structuur
 - api/ Laravel backend
-- apps/app/ Organizer SPA (hoofdapp)
-- apps/admin/ Super Admin SPA
+- apps/app/ Organizer + Platform Admin SPA (hoofdapp, super admin onder /platform/*)
 - apps/portal/ Externe portals (vrijwilliger, artiest, leverancier)
 
 ## Backend Regels (STRIKT VOLGEN)