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/domain/system/presentation/components/ChaptersStatusCard.vue

@@ -0,0 +1,36 @@
+<template>
+    <StatusCard title="Chapitres" :icon="DocumentTextIcon">
+        <div class="flex items-baseline gap-2 mb-3">
+            <span class="text-3xl font-bold text-gray-900 dark:text-white">{{ status.totalChapters }}</span>
+            <span class="text-sm text-gray-500">total</span>
+        </div>
+        <div class="mb-1 flex justify-between text-xs text-gray-500">
+            <span>{{ status.downloadedChapters }} téléchargés</span>
+            <span>{{ downloadedPercent }}%</span>
+        </div>
+        <div class="w-full h-2 bg-gray-200 dark:bg-gray-700 rounded-full overflow-hidden">
+            <div
+                class="h-full bg-green-500 rounded-full transition-all"
+                :style="{ width: downloadedPercent + '%' }" />
+        </div>
+        <p class="mt-1 text-xs text-gray-400">{{ status.pendingChapters }} en attente</p>
+    </StatusCard>
+</template>
+
+<script setup>
+import { computed } from 'vue';
+import { DocumentTextIcon } from '@heroicons/vue/24/outline';
+import StatusCard from './StatusCard.vue';
+
+const props = defineProps({
+    status: {
+        type: Object,
+        required: true,
+    },
+});
+
+const downloadedPercent = computed(() => {
+    if (!props.status.totalChapters) return 0;
+    return Math.round((props.status.downloadedChapters / props.status.totalChapters) * 100);
+});
+</script>