UI: update webview styles to match vscode 1.71+

## Problem

vscode 1.71 changed the vscode standard button style:

- corners are slightly curved

### New style

https://code.visualstudio.com/updates/v1_71#_rounded-buttons

### Old style

Buttons in our webviews look like:

The issue does not name a file or test; start by locating the webview button style definitions and compare them with the VS Code 1.71 rounded-button guidance linked in the report. Done means the webview buttons have the updated slightly curved corners and match the new reference style.

Written by the indexing model from the issue text.