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/types/api.ts

@@ -27,6 +27,7 @@ declare global {
     suppItems: Array<{ count: number; data: number }>
     startItems: Array<{ count: number; data: number }>
     pickrate: number
+    firstBacks?: FirstBackGroup[]
   }
 
   /**
@@ -56,6 +57,32 @@ declare global {
     championAlias: string
   }
 
+  /**
+   * Represents an item in a first back item set
+   */
+  interface FirstBackItemSetEntry {
+    itemId: number
+    count: number
+  }
+
+  /**
+   * Represents an item set (combination of items)
+   */
+  interface ItemSet {
+    items: FirstBackItemSetEntry[]
+    totalGold: number
+  }
+
+  /**
+   * Represents a grouped first back by item set
+   */
+  interface FirstBackGroup {
+    itemSet: ItemSet
+    count: number
+    pickrate: number
+    avgTimestamp: number
+  }
+
   /**
    * Represents lane-specific champion data
    */