feat(analytics): GA4 with GDPR Consent Mode v2

Google Analytics integration, off by default and GDPR-compliant for EU:

- src/lib/analytics/gtag.ts: typed event helpers + consent control. Every
  function is a safe no-op when NEXT_PUBLIC_GA_ID is unset.
- GoogleAnalytics.tsx: loads gtag.js with Consent Mode v2, all storage
  defaulting to "denied". anonymize_ip on, send_page_view off.
- ConsentBanner.tsx: on-brand cookie banner, localized to all 5 locales,
  persists choice for one year, flips analytics_storage to granted on accept.
- PageViewTracker.tsx: fires page_view on App Router client navigation
  (inside Suspense for useSearchParams).
- Key conversion events wired: ai_consultation_submitted (primary funnel
  goal) and ai_chat_opened.
- Consent strings added to messages/{en,it,vec,es,de}.json.

Build plumbing:
- NEXT_PUBLIC_GA_ID inlined at build time via Dockerfile ARG +
  docker-compose build.args (NEXT_PUBLIC_* must exist during next build,
  not just runtime).
- Nginx CSP extended to allow googletagmanager.com + google-analytics.com.
- env template documents NEXT_PUBLIC_GA_ID (empty = analytics disabled).

Verified: production build inlines the Measurement ID into the client
bundle; site builds cleanly both with and without the ID set.

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

docker-compose.yml

@@ -42,6 +42,10 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
+      args:
+        # NEXT_PUBLIC_GA_ID must be available at build time (Next.js inlines
+        # NEXT_PUBLIC_* into the client bundle). Sourced from .env on the host.
+        NEXT_PUBLIC_GA_ID: ${NEXT_PUBLIC_GA_ID:-}
     restart: always
     depends_on:
       postgres: