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/FirstBack.vue

@@ -0,0 +1,107 @@
+<script setup lang="ts">
+defineProps<{
+  firstBacks: FirstBackGroup[]
+  itemMap: Map<number, Item>
+}>()
+</script>
+
+<template>
+  <div v-if="firstBacks && firstBacks.length > 0" class="item-row">
+    <span class="item-row-label">First Back</span>
+    <div class="first-back-content">
+      <div v-for="(group, index) in firstBacks" :key="index" class="first-back-option">
+        <span class="gold-cost">{{ group.itemSet.totalGold }}g</span>
+        <div class="option-items">
+          <template v-for="item in group.itemSet.items" :key="item.itemId">
+            <div class="item-with-count">
+              <ItemIcon
+                v-if="itemMap.get(item.itemId)"
+                :item="itemMap.get(item.itemId)!"
+                :size="36"
+                class="item-cell"
+              />
+              <span v-if="item.count > 1" class="item-count">x{{ item.count }}</span>
+            </div>
+          </template>
+        </div>
+        <span class="pickrate">{{ (group.pickrate * 100).toFixed(0) }}%</span>
+      </div>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.item-row {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  max-width: 400px;
+}
+
+.item-row-label {
+  font-size: 0.8rem;
+  font-weight: 500;
+  color: var(--color-on-surface);
+  opacity: 0.6;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+}
+
+.first-back-content {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 16px;
+  overflow-x: hidden;
+}
+
+.first-back-option {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 2px;
+  flex-shrink: 0;
+}
+
+.option-items {
+  display: flex;
+  gap: 2px;
+}
+
+.item-with-count {
+  position: relative;
+  display: flex;
+  align-items: center;
+}
+
+.item-cell {
+  flex-shrink: 0;
+}
+
+.item-count {
+  position: absolute;
+  bottom: -1px;
+  right: -1px;
+  background: rgba(0, 0, 0, 0.85);
+  color: #fff;
+  font-size: 0.55rem;
+  font-weight: 600;
+  padding: 0 2px;
+  border-radius: 2px;
+  min-width: 12px;
+  text-align: center;
+}
+
+.pickrate {
+  font-size: 0.65rem;
+  color: var(--color-on-surface);
+  opacity: 0.6;
+  white-space: nowrap;
+}
+
+.gold-cost {
+  font-size: 0.6rem;
+  color: #ffd700;
+  opacity: 0.8;
+  white-space: nowrap;
+}
+</style>