docs: replace editor screenshot with demo video

[hashiiiii]

Summary

Replaces the static screenshot on the site's Unity Editor page with an MP4 conversion of the README's editor demo GIF, played as an autoplaying, looping, muted <video> inside the existing window frame.

Motivation

The editor page's only visual was one static frame while the README already shows the full animated flow. The GIF itself is 7 MB — too heavy for GitHub Pages — and GitHub's markdown sanitizer strips <video> tags, so the README keeps the GIF while the site gets the MP4 (734 KB, ~10× smaller).

Closes #148

Changes

• Add site/images/editor-01.mp4 (one-off ffmpeg conversion: H.264, yuv420p, +faststart, no audio, 1087×750 → 1086×750 for even dimensions)
• site/static/editor.html: swap the <img class="window-shot"> for <video autoplay loop muted playsinline> with the same accessibility text as an aria-label
• site/build.mjs: copy site/images/ into dist/images/; drop editor-window.png from the static copy list
• Delete site/static/editor-window.png (no remaining references)

Testing

$ ffprobe -v error -show_entries stream=codec_name,width,height,pix_fmt -of default=nw=1 site/images/editor-01.mp4
codec_name=h264
width=1086
height=750
pix_fmt=yuv420p

$ node site/build.mjs
site built at /Users/hashiiiii/workspace/PrefabLens/site/dist

$ ls -la site/dist/images/editor-01.mp4 && grep -n "editor-01.mp4" site/dist/editor.html
.rw-r--r--@ 734k hashiiiii 15 Jul 16:52 site/dist/images/editor-01.mp4
43:            src="images/editor-01.mp4"

Browser check against the built dist/ (Playwright, http://localhost:8321/editor.html): the video autoplays inside the window frame (paused=false, currentTime 8.65 → 9.85 over 1.2 s), loops, and fills the frame width.