Improve border bottom on active tab

Let me fix this for you, really like this project, smoother than the github one <3

Start by locating the active-tab UI entry point in the TypeScript frontend and compare its current bottom border with the attached image. The issue is done when the active tab's bottom border matches the requested appearance; no file or test is named, so the relevant component and verification step must first be identified.

索引モデルが issue の本文から書いたものです。