fix(web): align external API and service API buttons vertically in datasets header

[p2003722]

What this PR does / why we need it

Currently, in the datasets list header, the External Knowledge API button and the Service API button are not perfectly vertically aligned. This is caused by a difference in their DOM structure and flex properties:

1. The External Knowledge API button lacks an explicit h-6 class, making its height dependent on padding and line-height.
2. The Service API button is wrapped in a block-level <div> which interferes with the items-center flex alignment of the parent container.

This PR fixes the vertical alignment by:

• Adding h-6 to the External Knowledge API button to ensure a strict 24px height.
• Changing the Service API outer wrapper to <div className="flex items-center">.

Which issue(s) this PR fixes

N/A

[p2003722]

Hi @crazywoola @hyoban, just a tiny UI fix for the datasets header buttons' vertical alignment. Could you please take a quick look when you have time? Thanks!

[crazywoola]

Please attach a screenshot for frontend UI bugs.

[p2003722]

The two components in this area are not aligned vertically. @crazywoola