feat: add Google Analytics tracking for tickets and thank-you page

[dzienisz]

Implement Google Analytics Tracking for Ticket Sales

📝 Summary

This PR integrates GA4 (Google Analytics 4) ecommerce tracking to measure the conversion funnel—from the initial intent of clicking "Buy" to the successful purchase confirmation on the thank-you page.

🚀 Key Changes

• GA Utility Layer: Created src/utils/gtag.ts to provide a type-safe wrapper for the global gtag function. It includes safety checks to prevent errors during local development or if the GA script fails to load.
• Checkout Intent Tracking: Added the begin_checkout event to the "Get your ticket!" buttons in the TicketVariant component. This allows us to track which ticket types (Regular vs. VIP) users are interested in.
• Automated Purchase Tracking: Modified the ThankYou component to automatically trigger a purchase event. It extracts transaction details (Order ID, Product Name, Price, Currency, and Quantity) directly from the URL parameters provided by the checkout system.
• Developer Experience: Added proper TypeScript declarations for window.gtag to resolve linting errors and ensure type safety across the tracking implementation.

📊 Analytics Events

Event	Trigger	Captured Data
begin_checkout	Click on any "Get your ticket!" button	item_name, price, currency
purchase	Landing on the /thank-you page	transaction_id, value, currency, items (name, price, qty)

🛠 Testing Instructions

1. Checkout Intent: Open the site, open the console or Google Tag Assistant, and click "Get your ticket!". Verify the begin_checkout event fires.
2. Purchase: Navigate to a simulated success URL (which is also supported by the route validator):
   /thank-you?ec_order_id=TEST_123&ec_product=VIP&ec_price=512&ec_currency=PLN&ec_amount=1&ec_product_id=1&ec_product_uuid=1&ec_price_id=1
3. Confirm the purchase event appears in the GA4 Real-time dashboard with the correct values.