Add comprehensive visitor engagement tracking and scroll-to-top on navigation

Tracks scroll depth (25/50/75/90%), time on page, UTM parameters, outbound
link clicks, search queries, audio pause/completion/listen time, and 404s.
Logged-in study users are now tied to their visitor record and surfaced in
the admin recent visits table. Scroll position resets on every route change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AdminPage.tsx

@@ -540,6 +540,7 @@ export interface AdminStats {
       returningVisitor: boolean
       visitCount: number
       pageHistory?: Array<{ at: string; path: string; referrer?: string }>
+      studyUser?: { userId: string; username: string; displayName: string }
     }>
   }
   writeStatus: {
@@ -570,6 +571,15 @@ export interface AdminStats {
     byDay: Record<string, number>
     last30Days: Array<{ day: string; plays: number }>
   }>
+  engagement?: {
+    scrollDepth: Array<{ path: string; 25?: number; 50?: number; 75?: number; 90?: number }>
+    timeOnPage: Array<{ path: string; avgSeconds: number; count: number }>
+    topOutboundClicks: Array<{ url: string; count: number }>
+    topUTMSources: Array<{ source: string; count: number }>
+    topSearchQueries: Array<{ query: string; count: number }>
+    top404s: Array<{ path: string; count: number }>
+    audioEvents: Array<{ title: string; pauses: number; completions: number; totalListenSeconds: number }>
+  }
 }
 
 interface AdminAsset {