Add study activity tracking and refresh class header UI

src/app/api/activity/route.ts

@@ -0,0 +1,17 @@
+import { NextRequest, NextResponse } from "next/server";
+import { studyActivitySchema } from "@/lib/validation/activitySchemas";
+import * as activityService from "@/services/activityService";
+
+export async function GET() {
+  return NextResponse.json(await activityService.getActivitySummary());
+}
+
+export async function POST(request: NextRequest) {
+  const parsed = studyActivitySchema.safeParse(await request.json().catch(() => null));
+  if (!parsed.success) {
+    return NextResponse.json({ error: "Invalid activity type" }, { status: 400 });
+  }
+
+  await activityService.recordActivity(parsed.data.type);
+  return NextResponse.json({ success: true }, { status: 201 });
+}