Implement chain submissions API and update frontend to utilize new endpoint; enhance submissions page with feature flag for API selection, participant progress display, and improved filtering logic.

src/types/challenge.ts

@@ -244,3 +244,28 @@ export interface TestSubmissionResult {
   feedback?: string
 }
 
+// ========== Chain Submissions API ==========
+
+export interface ChainSubmissionsParticipant {
+  userId: string
+  nickname: string
+  completedTasks: number
+  totalTasks: number
+  progressPercent: number
+}
+
+export interface ChainSubmissionsResponse {
+  chain: {
+    id: string
+    name: string
+    tasks: Array<{ id: string; title: string }>
+  }
+  participants: ChainSubmissionsParticipant[]
+  submissions: ChallengeSubmission[]
+  pagination: {
+    total: number
+    limit: number
+    offset: number
+  }
+}
+