style(header): ajouter bouton toggle dark mode dans le header

feat(conversion): simplifier ConversionPage et brancher les toasts
style(manga): réécriture de la liste de résultats dans AddManga
chore(task): ajouter tâche conversion CBR→CBZ dans TASK.md

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

@@ -20,15 +20,36 @@
       </router-link>
       <SearchBar />
     </div>
+    <button
+      @click="toggleDarkMode"
+      class="mr-4 text-white p-2 hover:text-green-200 transition-colors"
+      :title="isDark ? 'Passer en mode clair' : 'Passer en mode sombre'"
+    >
+      <SunIcon v-if="isDark" class="h-6 w-6" />
+      <MoonIcon v-else class="h-6 w-6" />
+    </button>
   </header>
 </template>
 
 <script setup>
-import { Bars3Icon } from '@heroicons/vue/24/outline';
+import { computed } from 'vue';
+import { Bars3Icon, SunIcon, MoonIcon } from '@heroicons/vue/24/outline';
 import { useHeaderStore } from '../../stores/headerStore';
+import { useUserPreferencesStore } from '../../../domain/setting/application/store/userPreferencesStore';
 import SearchBar from './SearchBar.vue';
 
 const headerStore = useHeaderStore();
+const preferencesStore = useUserPreferencesStore();
+
+const isDark = computed(() => {
+    if (preferencesStore.theme === 'dark') return true;
+    if (preferencesStore.theme === 'light') return false;
+    return window.matchMedia('(prefers-color-scheme: dark)').matches;
+});
+
+function toggleDarkMode() {
+    preferencesStore.setTheme(isDark.value ? 'light' : 'dark');
+}
 
 defineProps({
   showMenuButton: {