fix: add left/right navigation buttons to carousel block

[StephenTangCook]

Summary

Fixes #65. The carousel block previously rendered as a horizontal scroll row with no navigation controls, while Slack's official Block Kit renderer overlays chevron buttons on the left/right edges of the row.

• Adds left/right chevron buttons overlaid on the carousel, matching Slack's caret-left / caret-right icon paths
• Buttons appear/disappear dynamically based on scroll position (left hides at the start, right hides at the end), using a scroll listener plus a ResizeObserver so resizing the container updates visibility
• Clicking scrolls the track by ~80% of its visible width using scrollBy({ behavior: "smooth" }), which naturally lands on the next snap point thanks to the existing snap-mandatory
• 8px tolerance on scroll-position checks so the existing px-1 track padding doesn't cause the left arrow to flash on load

Test plan

• pnpm lint (tsc) passes
• pnpm build produces clean ESM + CJS + d.ts output
• Playground renders the carousel with only the right chevron visible at initial scroll position, both chevrons after scrolling, and only the left chevron at the end
• Manual verification in a real browser that smooth-scroll + snap behave correctly (the headless preview I used did not support programmatic scrolling, but scrollBy is a standard browser API)

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