feat: add device scrollbar for all preview layouts

[itsreallythoshi]

📓 Referenced Issue

Fixes #1475

ℹ️ About the PR

Adds a collapsible device navigation bar that floats at the bottom of the
preview area for Column, Flex, and Masonry layouts.

How it works:

• An IntersectionObserver tracks which devices are visible in the scroll container
• All devices with >30% visibility are highlighted simultaneously
• Clicking a thumbnail scrolls to that device (horizontal for Column, vertical for Flex/Masonry)
• In Column layout, the carousel also supports drag-to-pan
• The bar is collapsible via a toggle button

Design decisions:

• Uses the app's existing slate color palette (matches Button component active states)
• No third-party libraries added

🖼️ Testing Scenarios / Screenshots

Column layout - scrolling, multiple highlights, clicking thumbnail

Flex layout - same

Masonry layout - same

Collapse/expand the bar

Appropriate light and dark mode

[CLAassistant]

All committers have signed the CLA.