docs: replace waitlist form with Loops.so integration

[BYK]

Summary

Replace the FormSubmit.co waitlist form on the website with a Loops.so newsletter form integration.

Changes

Backend

• FormSubmit.co Loops.so (https://app.loops.so/api/newsletter-form/...)
• Content-Type: application/x-www-form-urlencoded (Loops requirement)
• Progressive enhancement: action + method on <form> so native POST works without JS

UI/UX

• Three-view state system: form (default), success, error — only one visible at a time via .cta-form-view/.show toggle
• Retry support: error view shows a "Try Again" button that restores the form with email preserved
• Loading state: button shows "Sending..." and disables during fetch, restores in finally
• Rate limiting: 60s cooldown via localStorage timestamp (matches Loops embed behavior)

Robustness

• res.json() failure (non-JSON response like 502) caught cleanly — falls to network error
• localStorage wrapped in try/catch for Safari private browsing compatibility
• Rate-limit timestamp only recorded on success (not on API error responses)
• Email input preserved on error (only cleared on success)
• "Email already on list" treated as success

Accessibility

• aria-live="polite" on success view for screen reader announcements
• role="alert" on error view for immediate screen reader notification
• Focus management: retry button focused on error, email input focused on retry

Styling

• All inline styles from Loops embed stripped — uses existing CSS variables (--g0, --c0, --c2, etc.)
• No Inter font import — inherits site's DM Sans
• New classes: .cta-form-view, .cta-error, .cta-back, .cta-submit:disabled
• Mobile responsive: .cta-back added to full-width rule
• Editorial aesthetic preserved (2px border-radius, uppercase letter-spacing, cream/green palette)