feat(ai): add custom scrollbar to block-chat

[jbecke]

Motivation

• Provide a consistent custom scrollbar for the block-chat message area and hide the native scrollbar to match existing UI patterns.

Description

• Import CustomScrollbar from @core/component/CustomScrollbar and add a scrollRef signal via createSignal to hold the scroll container element.
• Add ref={setScrollRef} and scrollbar-hidden to the chat scroll container and make the container relative to host the overlay scrollbar.
• Render <CustomScrollbar scrollContainer={scrollRef} /> next to the chat container to wire the custom scrollbar to the chat messages.

Testing

• No automated tests were run for this change.

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Codex Task

[github-actions]

Preview: https://codex-add-custom-scrollbar-to--v9vamo.preview.macro.com/app (ce16475)

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.