Fix chat panel horizontal scroll + padding

[quan0715]

• Enable horizontal scrollbar in ScrollArea (opt-in via prop)\n- Use horizontal+vertical scrollbars in chat message panel\n- Add left/right padding for chat messages

[Copilot]

Pull request overview

This PR updates the frontend scrolling utility and chat UI to better handle wide content (e.g., long code lines) by allowing opt-in horizontal scrollbars and adding more consistent horizontal padding in the chat message list.

Changes:

• Added an opt-in scrollbar mode to the shared ScrollArea component (vertical | horizontal | both).
• Enabled both horizontal and vertical scrollbars in the chat messages panel.
• Adjusted chat message container padding from p-3 to px-4 py-3.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
frontend/src/components/ui/scroll-area.tsx	Adds configurable scrollbar rendering (vertical/horizontal/both) while keeping default behavior vertical-only.
frontend/src/components/chat/ChatPanel.tsx	Enables both scrollbars for the message list and tweaks padding to improve layout/spacing.

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