feat: add a minimal separator between pinned apps and unpinned apps in the explore page

[kurokobo]

Summary

This PR adds a minimal separator between Pinned Apps and Unpinned Apps in the Explore page. The separator is displayed only if there are both pinned apps and unpinned apps.

Tested as follows:

• Pin one or more apps and then ensure the separator is displayed
• Unpin all apps and then ensure the separator is not displayed
• Pill all apps and then ensure the separator is not displayed

Fixes #10869

Tip

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Screenshots

There are 5 pinned apps on the top.

Before	After

Checklist

Important

Please review the checklist below before submitting your pull request.

• 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 dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods