Convert animated GIFs to MP4 to cut file size (#119)

[jonfroehlich]

Re-encodes the GIFs referenced from published lessons to H.264 MP4 and removes the originals, per #119. Builds on #120 (movies/→videos/), following the agreed "rename first, then convert" order.

What changed

• 27 referenced GIFs → MP4 (libx264 -crf 23 -pix_fmt yuv420p -movflags +faststart -an, with an even-dimension scale filter), and each ![alt](x.gif) swapped to a muted autoplay/loop <video aria-label="alt">. Table cells use the single-line <video src=…> form so they stay inside the table.
• 50 GIFs deleted — the converted originals plus commented-out and never-referenced orphans (full list in the diff).
• 2 same-name source MP4s preserved over GIF transcodes. Where the repo already had a same-name .mp4, that source is used instead of re-deriving from the degraded GIF. The oversized 13.6 MB Arduino_LEDBlink_Pin3.mp4 was re-encoded to 1.2 MB at source quality (it was even bigger than its GIF).
• OG social cards fixed. 8 arduino pages set image: to a hero GIF that's now a <video>; social crawlers can't render video, so I regenerated static posters with generate_og_posters.py and repointed image: at assets/og/<page>.jpg. Updated the website-dev.md front-matter example to led-fade's real poster path to stay accurate.

Out of scope

signals/ GIFs are untouched — those notebooks are migrating to makeabilitylab/signals (#116).

Net size impact

~119 MB removed: 112.7 MB of GIFs + 12.4 MB from the LEDBlink re-encode, against 5.8 MB of new MP4s + posters.

Verification

• bundle exec jekyll build clean (Jekyll 4)
• Built _site has zero .gif refs in arduino/electronics/esp32/sensors; all MP4s + posters resolve; heroes render as <video>
• All new <video> elements carry aria-label (media-a11y gate) — CI's html-proofer link-check + media-a11y are the backstop

Note / possible follow-up

The electronics/assets/gifs/ dir now holds .mp4 files (the dir name is a slight misnomer). Left as-is to keep this PR focused; could be renamed in a small follow-up if desired.

🤖 Generated with Claude Code