Fix API_BASE_URL to use runtime configuration

CRITICAL FIX: Checkout was failing because API calls went to localhost

The checkoutClient was hardcoded to use import.meta.env.VITE_API_BASE_URL
which is build-time only. In production, this defaulted to localhost:3001
instead of the actual backend URL.

Changed to read from window.env.API_BASE_URL (runtime config) first,
then fall back to build-time config. This matches the pattern used
in src/utils/config.ts.

Frontend will now correctly connect to:
- Development: http://localhost:3001
- Production: https://puffinoffset.com/api (from .env file)

Fixes network error: "Failed to fetch" when creating checkout session

🔒 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/api/checkoutClient.ts

@@ -1,7 +1,18 @@
 import axios from 'axios';
 import { logger } from '../utils/logger';
 
-const API_BASE_URL = import.meta.env.VITE_API_BASE_URL || 'http://localhost:3001';
+// Get API base URL from runtime config (window.env) or build-time config
+const getApiBaseUrl = (): string => {
+  // Check window.env first (runtime config from env.sh)
+  if (typeof window !== 'undefined' && window.env?.API_BASE_URL) {
+    return window.env.API_BASE_URL;
+  }
+
+  // Fall back to build-time env or default
+  return import.meta.env.VITE_API_BASE_URL || 'http://localhost:3001';
+};
+
+const API_BASE_URL = getApiBaseUrl();
 
 export interface CreateCheckoutSessionParams {
   tons: number;