Add Яндекс.Метрика funnel goals: signup_free + gate_view (env-driven counter)

- window.ym reachGoal 'signup_free' on registration success
- GateViewTracker client component fires 'gate_view' on tripwire gate mount
- counter id via NEXT_PUBLIC_METRIKA_COUNTER_ID (build-time)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/student/gate-view-tracker.tsx

@@ -0,0 +1,20 @@
+"use client";
+
+import { useEffect } from "react";
+
+declare global {
+  interface Window {
+    ym?: (counterId: number, action: string, goal: string) => void;
+  }
+}
+
+// Фиксирует показ gate-лестницы в Яндекс.Метрике (цель gate_view).
+export function GateViewTracker() {
+  useEffect(() => {
+    const c = Number(process.env.NEXT_PUBLIC_METRIKA_COUNTER_ID);
+    if (c && typeof window !== "undefined" && typeof window.ym === "function") {
+      window.ym(c, "reachGoal", "gate_view");
+    }
+  }, []);
+  return null;
+}