feat: relocate scrollbar to navigation, add scroll-UI idle fade

[bertpl]

Summary

• Moved scrollbar from per-slide containers to .navigation, wrapped with snap control in a shared .scroll-ui div.
• Idle fade on the wrapper: 1s delay, 0.5s fade-out, instant fade-in on any scroll interaction, hover, or snap control use.
• CSS opacity on the wrapper multiplies through to all children — no duplicate opacity management.
• JS ScrollManager and SnapManager refactored to reference the single global scrollbar element instead of querying per-container.

Test plan

• 646 Python + 98 JS tests pass
• Built worked example — scrollbar and snap control visible on scrollable slides
• Idle fade works: stops scrolling → 1s → fade out; scroll again → instant fade in
• Hover over scroll UI resets the idle timer
• Deck view: no scrollbar visible
• Slide changes reset the idle timer

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