Refactor Enter to pure CSS web component

[gavmck]

Summary

• Converts the dmd-enter component from a Lit component to a pure CSS web component, removing all JavaScript (LitElement, IntersectionObserver, render logic)
• Uses animation-timeline: view() and animation-range for scroll-triggered entrance animations instead of IntersectionObserver
• Uses CSS attr(delay s, 0s) to read the delay value directly from the HTML attribute

Browser support

• animation-timeline: view() — Chrome 115+, Edge 115+, Firefox 131+. No Safari support yet.
• attr() with type coercion — Chrome/Edge only. Delay gracefully degrades (no delay applied in unsupported browsers).

Test plan

• Verify entrance animations (fade, fade-in-up, boing) play on mount
• Verify enter-on-scroll triggers animation when scrolling element into view
• Verify delay attribute delays the animation start
• Verify fill attribute applies grid display
• Test in Chrome/Edge and verify scroll-driven + delay behaviour
• Test in Firefox and verify scroll-driven animation works
• Test in Safari and verify graceful degradation (animation plays immediately without scroll trigger)