feat(system): page Status avec endpoint API Platform et composants Vue

- Nouveau domaine System/Domain/Model/SystemStatus (value object)
- QueryHandler agrégeant métriques mangas, chapitres, jobs (global/24h/7j), stockage et sources
- Endpoint GET /api/system/status via API Platform (singleton)
- Calcul de l'espace disque par RecursiveDirectoryIterator sur public/images
- Page Vue /system/status avec 6 cards (Mangas, Chapitres, Jobs, Stockage, Sources, Système)
- Nettoyage du router : suppression des PlaceholderComponent et routes placeholder
- Sidebar : suppression des entrées sans page réelle

assets/vue/app/shared/components/layout/Sidebar.vue

@@ -78,11 +78,9 @@ import MenuGroup from './sidebar/MenuGroup.vue';
         {
             icon: Cog6ToothIcon,
             text: 'Paramètres',
-            to: '/settings',
+            to: '/settings/scrappers',
             id: 'settings',
             subItems: [
-                { icon: null, text: 'Général', to: '/settings/general' },
-                { icon: null, text: 'Dossiers', to: '/settings/folders' },
                 { icon: null, text: 'Scrappers', to: '/settings/scrappers' },
                 { icon: null, text: 'UI', to: '/settings/ui' }
             ]
@@ -90,13 +88,11 @@ import MenuGroup from './sidebar/MenuGroup.vue';
         {
             icon: ComputerDesktopIcon,
             text: 'Système',
-            to: '/system',
+            to: '/system/status',
             id: 'system',
             subItems: [
                 { icon: null, text: 'Status', to: '/system/status' },
-                { icon: null, text: 'Backup', to: '/system/backup' },
                 { icon: null, text: 'Logs', to: '/system/logs' },
-                { icon: null, text: 'Updates', to: '/system/updates' }
             ]
         }
     ];