fix: add guard in clPhotoswipe and correct Mailchimp URL

[raja2576]

This PR fixes two issues in the writers-program frontend JavaScript code:

1. ✅ PhotoSwipe Gallery Bug: Added a guard clause to safely handle cases where data-size is missing from thumbnail links. This prevents .split() from throwing runtime errors.
2. ✅ Mailchimp Integration: Replaced incorrect post URL with the proper post-json format required for Mailchimp’s AJAX submission to work correctly.

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Changes Made

• Modified clPhotoswipe function to skip items with undefined data-size
• Updated the mailChimpURL in the config to use post-json?...&c=?
• Minor code clean-up and safe DOM access

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Why This is Important

• Fixes a JavaScript crash that occurs when data-size is not defined on a gallery item
• Restores working Mailchimp newsletter form for users subscribing via the frontend

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Testing

• ✅ Manually tested the gallery: no errors in console when clicking thumbnails
• ✅ Mailchimp form submits successfully with a valid email
• ✅ No regressions observed in animations or scrolling

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Type of Contribution

• Bug fix 🐛
• Code enhancement 🔧