feat: first back recording and display (#12)

Record first backs, group them by item sets and show the most popular ones, with gold and %, in the frontend.

frontend/components/build/Viewer.vue

@@ -3,6 +3,7 @@ import { getCoreItems, getLateGameItems } from '~/utils/buildHelpers'
 import BuildVariantSelector from '~/components/build/BuildVariantSelector.vue'
 import CompactRuneSelector from '~/components/build/CompactRuneSelector.vue'
 import ItemRow from '~/components/build/ItemRow.vue'
+import FirstBack from '~/components/build/FirstBack.vue'
 
 const props = defineProps<{
   builds: Builds
@@ -122,6 +123,13 @@ function selectBuild(index: number): void {
           />
         </div>
 
+        <!-- First Back Section -->
+        <FirstBack
+          v-if="currentBuild.firstBacks && currentBuild.firstBacks.length > 0"
+          :first-backs="currentBuild.firstBacks"
+          :item-map="itemMap"
+        />
+
         <!-- Core Items Tree (children of start item) -->
         <div v-if="currentBuild.items?.children?.length" class="item-row">
           <span class="item-row-label">Core</span>