Add study activity tracking and refresh class header UI

prisma/migrations/20260712113000_add_study_activity/migration.sql

@@ -0,0 +1,9 @@
+-- CreateTable
+CREATE TABLE "StudyActivity" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "type" TEXT NOT NULL,
+    "occurredAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+-- CreateIndex
+CREATE INDEX "StudyActivity_occurredAt_idx" ON "StudyActivity"("occurredAt");

prisma/schema.prisma

@@ -141,6 +141,14 @@ model Setting {
   value String
 }
 
+model StudyActivity {
+  id         String   @id @default(uuid())
+  type       String // "FLASHCARD" | "QUIZ_QUESTION"
+  occurredAt DateTime @default(now())
+
+  @@index([occurredAt])
+}
+
 model MaterialGroup {
   id        String   @id @default(uuid())
   classId   String