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/index.vue

@@ -91,7 +91,7 @@
             :key="chapter.path"
           >
             <NuxtLink
-              :to="`/modele-eco/${chapter.stem}`"
+              :to="`/modele-eco/${chapter.stem?.split('/').pop()}`"
               class="card-surface flex items-start gap-4 group"
             >
               <span class="font-mono text-2xl font-bold text-primary/30 leading-none mt-1 w-10 text-right flex-shrink-0">
@@ -109,7 +109,7 @@
                     <span class="i-lucide-clock inline-block h-3 w-3 mr-1 align-middle" />
                     {{ chapter.readingTime }}
                   </span>
-                  <SongBadges :chapter-slug="chapter.stem!" />
+                  <SongBadges :chapter-slug="chapter.stem?.split('/').pop() ?? ''" />
                 </div>
               </div>
               <div class="i-lucide-chevron-right h-5 w-5 text-white/20 group-hover:text-primary/60 transition-colors flex-shrink-0 mt-2" />