Fix 404 chapitres : stem Nuxt Content inclut le dossier parent

Le `chapter.stem` de Nuxt Content renvoie `book/01-introduction` et non
`01-introduction`. Extraction du slug final via `.split('/').pop()` dans
les liens et la navigation prev/next.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/pages/modele-eco/[slug].vue

@@ -14,7 +14,7 @@
     <nav class="mt-16 flex items-center justify-between border-t border-white/8 pt-8">
       <NuxtLink
         v-if="prevChapter"
-        :to="`/modele-eco/${prevChapter.stem}`"
+        :to="`/modele-eco/${prevChapter.stem?.split('/').pop()}`"
         class="btn-ghost gap-2"
       >
         <div class="i-lucide-arrow-left h-4 w-4" />
@@ -24,7 +24,7 @@
 
       <NuxtLink
         v-if="nextChapter"
-        :to="`/modele-eco/${nextChapter.stem}`"
+        :to="`/modele-eco/${nextChapter.stem?.split('/').pop()}`"
         class="btn-ghost gap-2"
       >
         <span class="text-sm">{{ nextChapter.title }}</span>
@@ -64,7 +64,7 @@ const { data: allChapters } = await useAsyncData('book-nav', () =>
 )
 
 const currentIndex = computed(() =>
-  allChapters.value?.findIndex(c => c.stem === slug) ?? -1,
+  allChapters.value?.findIndex(c => c.stem?.split('/').pop() === slug) ?? -1,
 )
 
 const prevChapter = computed(() => {