🔒 Fix: move Systeme.io API call to server-side endpoint to hide API key

[lsb11]

🎯 What: The kdsymoc8fwhwwc604vrofp4jt0rb5bjosmzs5dqd4r8suo4avx4xsik4ixqvwgo4 Systeme.io API key was hardcoded in src/pages/shopify-automation-guides.astro client-side Javascript.

⚠️ Risk: The API key was exposed in the browser to anyone viewing the page source. This could have allowed malicious actors to make unauthorized API calls to the store's systeme.io instance, potentially adding/extracting contact info or modifying data.

🛡️ Solution: Removed the hardcoded API key from the client-side code and redirected the fetch call to a new server-side endpoint (/api/submit-guide). Implemented a Cloudflare Pages API function at functions/api/submit-guide.js that retrieves the key securely via the SYSTEME_IO_API_KEY environment variable and proxies the request to the systeme.io API. Included comprehensive unit tests (submit-guide.test.js) to verify validation logic and correct downstream API interaction.

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PR created automatically by Jules for task 2111939439338103177 started by @lsb11

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