fix: fix submit email button on what's new page

[betzlermeow]

Description

• Fix the broken "Submit" email button on the What's New page (/whats-new)
• Root cause: Fern's MDX renderer strips JSX event handlers (onSubmit, onClick), so the form's validation and submission logic was never attached to the DOM. The form fell through to a native HTML POST that silently redirected back to the same page with no user feedback.
• Fix: Move all form logic to custom.js using plain DOM addEventListener('submit', ...), which Fern does not strip
• Replace inline styles in overview.mdx with CSS classes in styles.css for proper styling (including dark mode)
• Add a feedback message div that shows success/error states inline after submission
• Handle Customer.io's 302 redirect response using fetch() with redirect: 'manual' and opaqueredirect type check
• Add MutationObserver to re-attach handler on SPA route changes (Fern uses client-side routing)
• Guard against duplicate handler attachment with data-enhanced attribute

Closes PRO-2451

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work
• Navigate to the What's New page (/whats-new)
• Enter a valid email and click Submit -- should see "Thanks for subscribing!" success message
• Enter an invalid email and click Submit -- should see "Please enter a valid email address." error message
• Verify button shows "Submitting..." loading state during submission
• Verify dark mode styling works correctly for the form
• Run tests with node fern/custom.spec.js (39 tests, 0 dependencies)

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