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/app/(auth)/register/register-form.tsx

@@ -5,6 +5,7 @@ import Link from "next/link";
 
 declare global {
   interface Window {
+    ym?: (counterId: number, action: string, goal: string) => void;
     turnstile?: {
       render: (
         container: HTMLElement,
@@ -161,6 +162,11 @@ export function RegisterForm({ showTermsCheckbox, privacyPolicyUrl, termsUrl, of
       return;
     }
 
+    // Яндекс.Метрика: цель «бесплатная регистрация»
+    const mc = Number(process.env.NEXT_PUBLIC_METRIKA_COUNTER_ID);
+    if (mc && typeof window !== "undefined" && typeof window.ym === "function") {
+      window.ym(mc, "reachGoal", "signup_free");
+    }
     setSuccess(true);
     setLoading(false);
   }