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/tripwire-gate-banner.tsx

@@ -2,6 +2,8 @@
 // основной — трипвайр «Второй мозг для предпринимателя», вторичный — полный курс.
 // Server component, ДС-2 (Aubade).
 
+import { GateViewTracker } from "./gate-view-tracker";
+
 const TRIPWIRE_URL = "https://second-brain.ru/entrepreneur";
 const FULL_COURSE_URL = "https://obsidian.second-brain.ru/";
 
@@ -11,6 +13,7 @@ export function TripwireGateBanner() {
       className="card-aubade p-6 mt-10"
       style={{ background: "var(--accent)", boxShadow: "4px 4px 0 0 var(--foreground)" }}
     >
+      <GateViewTracker />
       <p
         className="text-xs font-bold uppercase tracking-widest mb-2"
         style={{ color: "var(--muted-foreground)" }}