feat(web): add base ui scroll area primitive

[lyzno1]

• feat: base ui scroll bar
• style: add stories and focus ring
• update stories
• update stories

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Summary

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

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[Copilot]

Pull request overview

This PR introduces a new base/ui ScrollArea primitive wrapper around @base-ui/react/scroll-area, along with Storybook coverage and a Vitest test suite, to standardize scrollable panel compositions in the web UI.

Changes:

• Added ScrollArea compound exports and design-system className defaults (viewport, scrollbar, thumb, corner).
• Added comprehensive Storybook stories demonstrating common Dify scroll surface compositions.
• Added unit tests validating rendering and default className behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
web/app/components/base/ui/scroll-area/index.tsx	Introduces the ScrollArea wrapper/exports and default styling tokens.
web/app/components/base/ui/scroll-area/index.stories.tsx	Adds multiple Storybook compositions for vertical, horizontal, overlay, and corner cases.
web/app/components/base/ui/scroll-area/__tests__/index.spec.tsx	Adds tests for compound rendering and default className/orientation behavior.

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