fix(constructeurs): improve search filtering and duplicate prevention

Switch ConstructeurSelect to client-side filtering instead of debounced
API calls. Add duplicate name check before creating a new constructeur
in both ConstructeurSelect and the constructeurs page.

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

app/pages/constructeurs.vue

@@ -195,8 +195,18 @@ const closeModal = () => {
 }
 
 const saveConstructeur = async () => {
+  const trimmedName = form.value.name.trim()
+  const duplicate = constructeurs.value.find(
+    (c) => c.name.toLowerCase() === trimmedName.toLowerCase()
+      && c.id !== editingConstructeur.value?.id,
+  )
+  if (duplicate) {
+    showError(`Un fournisseur "${duplicate.name}" existe déjà.`)
+    return
+  }
+
   saving.value = true
-  const payload = { ...form.value }
+  const payload = { ...form.value, name: trimmedName }
   if (!payload.email) { delete payload.email }
   if (!payload.phone) { delete payload.phone }
   let result