Upgrade website to Gatsby v5 and node v24

[lalitpatel]

Summary

Upgrade the site from Gatsby 3 to Gatsby 5.16.1, keeping React on 18.3.1 for a lower-risk migration.

This also updates the MDX rendering path for Gatsby 5, moves SEO metadata to Gatsby’s built-in Head API, keeps current trailing-slash URL behavior, and switches local image usage to Gatsby’s image pipeline.

Changes

• Upgrade Gatsby, React, React DOM, MDX, and Gatsby-owned plugins to Gatsby 5-compatible versions.
• Add package-lock.json for reproducible npm installs.
• Set trailingSlash: 'never' to preserve existing URL behavior.
• Migrate Gatsby GraphQL sort syntax to the Gatsby 5 shape.
• Replace React Helmet usage with Gatsby Head exports.
• Convert local image rendering to gatsby-plugin-image / GatsbyImage.
• Remove unused Markdown remark plugins and app-level peer dependencies.
• Preserve Stripe Payment Link donation and membership flows.
• Deleted .eslintrc because the repo already uses eslint.config.js

Verification

nvm use
npm install --legacy-peer-deps
npm run lint
gatsby clean && gatsby build && gatsby serve