Add challenges background preference support: Extend site preferences and related components to include challengesBackground, update API and UI to handle new background image settings for challenges.

lib/preferences.ts

@@ -1,7 +1,7 @@
 import { sitePreferencesService } from "@/services/preferences/site-preferences.service";
 
 export async function getBackgroundImage(
-  page: "home" | "events" | "leaderboard",
+  page: "home" | "events" | "leaderboard" | "challenges",
   defaultImage: string
 ): Promise<string> {
   return sitePreferencesService.getBackgroundImage(page, defaultImage);