refactor(ui) : improve styling, layout and responsive across all components

Rework CSS theme (app.css), navbar layout, dashboard page, machine detail,
catalog pages, and various form/display components for better consistency
and mobile responsiveness.

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

app/components/PieceModelStructureEditor.vue

@@ -10,7 +10,7 @@
         </p>
       </header>
 
-      <p v-if="!products.length" class="text-xs text-gray-500">
+      <p v-if="!products.length" class="text-xs text-base-content/50">
         Aucun produit défini.
       </p>
 
@@ -76,7 +76,7 @@
         Champs personnalisés
       </h3>
 
-      <p v-if="!fields.length" class="text-xs text-gray-500">
+      <p v-if="!fields.length" class="text-xs text-base-content/50">
         Aucun champ personnalisé n'a encore été défini.
       </p>