fix: add tabIndex on base code block

[lawreka]

Summary

• different linters (biome vs. axe) are fighting - client app (axe) says not having tabIndex here is an accessibility regressions, so repressing the biome error

Test Plan

• no visible changes, code blocks should be more keyboard-accessible

---

Note

Low Risk
Small, isolated accessibility change that only affects focus behavior on the code block container.

Overview
Adds tabIndex={0} to the BaseCodeBlock root container so scrollable code blocks can receive keyboard focus, aligning with axe accessibility expectations.

Includes a biome lint suppression comment to allow the non-interactive tabindex in this specific case.

^{Written by Cursor Bugbot for commit 1bc1c23. This will update automatically on new commits. Configure here.}