fix(website): rewind landing editor scroll when a scene renders

[zantvoort]

Follow-up to #188, which gave the landing editor's horizontal scrollers opaque backgrounds but did not fix the reported symptom on iOS: scenes rendering with the first ~13 characters of every line cut off (screenshot in the report showed a uniform 13-character offset on all 15 lines, with the 24px left padding gone too).

Actual cause: a retained scroll offset, not paint. Any touch pan on the code block — including an accidental diagonal swipe while scrolling the page, which is near-unavoidable on a phone — leaves #code scrolled right. On desktop the next scene self-heals: the typewriter's near-empty first render collapses scrollWidth, and the browser clamps scrollLeft back to 0. iOS Safari does not clamp the offset of a composited scroller when its content shrinks, so the stale offset survives every innerHTML replacement and each new scene types into a shifted viewport. Nothing in the page ever reset the scroll position.

Fix: reset scrollLeft explicitly at the three points where scroller content is replaced:

• when a scene starts typing (covers auto-advance, tab clicks, and the reduced-motion path),
• when the finished snippet is rendered, so a mid-typing pan can't leave the resting scene shifted through its dwell,
• when the Show-SQL panel content is replaced.

Deliberate panning during a scene's dwell is untouched — the reset only happens when content changes.

The #188 background rules are kept; they're harmless and guard the compositing-paint case independently.

Verified with npm run build (only pre-existing broken-anchor warnings in old snapshots).