do to formatters

apps/website/lib/formatters/ActivityLevelFormatter.ts

@@ -0,0 +1,33 @@
+/**
+ * ActivityLevelDisplay
+ *
+ * Deterministic mapping of engagement rates to activity level labels.
+ */
+
+export class ActivityLevelFormatter {
+  /**
+   * Maps engagement rate to activity level label.
+   */
+  static levelLabel(engagementRate: number): string {
+    if (engagementRate < 20) {
+      return 'Low';
+    } else if (engagementRate < 50) {
+      return 'Medium';
+    } else {
+      return 'High';
+    }
+  }
+
+  /**
+   * Maps engagement rate to activity level value.
+   */
+  static levelValue(engagementRate: number): 'low' | 'medium' | 'high' {
+    if (engagementRate < 20) {
+      return 'low';
+    } else if (engagementRate < 50) {
+      return 'medium';
+    } else {
+      return 'high';
+    }
+  }
+}