Transform-based slide deck: hide scrollbar, true slide-by-slide nav

[hansjohn819-commits]

Why

Two UX issues with the scroll-based deck during real presentations:

1. Scrollbar shows up in fullscreen. When presenting (F11), the
   browser's right-edge scrollbar is a constant visual distraction —
   it makes the page feel like "a webpage that scrolls" rather than
   "slides advancing". scroll-snap-type doesn't hide it.

2. Trackpad fine-scrolls can land between slides. Even with
   scroll-snap-type: y mandatory, the visual feel is "scrolling that
   snapped" rather than "the next slide came in".

What changed

All slides now live inside a single <div class="deck"> container.
Navigation moves the deck via transform: translateY(-idx * 100vh)
instead of scrolling. body has overflow: hidden so no scrollbar
ever shows. Smooth between-slide motion comes from a CSS transition
on .deck, and is disabled under prefers-reduced-motion.

User-facing navigation is unchanged: arrow keys, space, mouse wheel,
click slide, click nav dots all advance the active slide.

Files

• viewport-base.css — html/body overflow: hidden; new
  .deck rule with transition; removed scroll-snap-* and .slide
scroll-snap-align; prefers-reduced-motion now also disables
  the deck transition.
• html-template.md — body skeleton wraps slides in
  <div class="deck" id="deck">; SlidePresentation controller
  rewritten to drive the transform via goTo(). The old
  Intersection Observer is replaced by a direct activate(idx) call
  that marks the active slide visible and updates dots / progress.
• SKILL.md — Phase 5 navigation summary updated.

Testing

• Verified in Chrome / Edge on Windows at 1080p and 4K
• Fullscreen (F11) shows no scrollbar
• All four nav methods (keyboard / wheel / click slide / click dot) advance correctly
• Reveal animations still fire on slide entry via .slide.visible .reveal
• prefers-reduced-motion: reduce disables the deck transition