feat(ui) : clickable entity links, site→machines links, DataTable fixedLayout

- Add NuxtLink on component/piece/product names in machine hierarchy
  (using composantId, pieceId, product.id)
- Make site machine count badges clickable → /machines?sites={id}
- Add opt-in fixedLayout prop and minWidth to DataTable columns

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app/components/PieceItem.vue

@@ -29,7 +29,15 @@
         </button>
         <div class="flex-1 min-w-0">
           <h3 class="text-lg font-semibold" :class="{ 'text-error': piece._emptySlot || piece.pendingEntity }">
-            {{ pieceData.name }}
+            <NuxtLink
+              v-if="!isEditMode && !piece.pendingEntity && !piece._emptySlot && piece.pieceId"
+              :to="`/piece/${piece.pieceId}`"
+              class="hover:underline hover:text-primary transition-colors"
+              @click.stop
+            >
+              {{ pieceData.name }}
+            </NuxtLink>
+            <template v-else>{{ pieceData.name }}</template>
             <span v-if="piece._emptySlot" class="text-sm font-semibold text-error ml-1">— manquant</span>
             <button
               v-if="piece.pendingEntity"