feat: bring the KakaoPay support button over to Astro

[devFancy]

Background

The KakaoPay support button that sat in the Jekyll header did not come across to Astro. Both images are still in public/assets/img/etc/kakaopay/.

Goal

Bring the button back, and fix the problems in the old implementation while doing it.

Plan

1. Decide where it goes. Above Recommend at the end of a post collides least with the current layout.
2. Rewrite the modal.
3. Show it on desktop only. A QR code cannot be scanned by the phone displaying it.

Verify

• The QR code is readable in dark mode.
• ESC closes the modal and focus stays inside it.

Notes

Do not port js/kakaopay.js as is.

• It overwrites window.onclick and window.onload wholesale. Use addEventListener instead.
• Replace the userAgent match with matchMedia('(hover: none)').
• It has no ESC handling, no focus trap and no aria-modal.
• buymeacoffee was already commented out in Jekyll. Do not bring it back.
• The original is in the jekyll-final tag.