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/common/StructureSkeletonPreview.vue

@@ -93,7 +93,7 @@
         <!-- Empty state: component variant -->
         <p
           v-if="variant === 'component' && showEmptyState && !customFields.length && !pieces.length && !products.length && !subcomponents.length"
-          class="text-xs text-gray-500"
+          class="text-xs text-base-content/50"
         >
           Ce squelette ne définit pas encore de pièces, sous-composants ou valeurs par défaut.
         </p>