feat: add reusable Modal component with ARIA accessibility closes #71

[AYo101o]

♿ Reusable Modal Component with ARIA Accessibility

Closes #71

What this PR does

Adds a reusable <Modal /> base component at components/ui/Modal.tsx that any dev can use to build modals across the project, styled consistently with Nevo's dark theme.

Features

• Accessible by default — role="dialog", aria-modal="true", aria-labelledby for the title, aria-describedby for the subtitle, and aria-label on the close button
• Focus trap — Tab and Shift+Tab cycle stays within the modal when open
• Keyboard support — Escape key closes the modal
• Focus restoration — returns focus to the trigger element on close
• Backdrop click to close — with optional disableBackdropClose prop
• Flexible API — accepts title, subtitle, children, footer, and size (sm | md | lg | xl) props
• Fully responsive across all screen sizes
• Styled with Nevo's color palette (#0a1428 bg, #1fe4ff cyan accent, #10b981 green)

Usage

<Modal
  isOpen={open}
  onClose={() => setOpen(false)}
  title="Confirm Action"
  subtitle="This cannot be undone."
  size="md"
  footer={<button onClick={() => setOpen(false)}>Confirm</button>}
>
  <p>Modal content here.</p>
</Modal>