Fix mobile overflow on install tabs

[ded-furby]

Summary

• allow the install tab bar to scroll horizontally on small screens
• prevent individual tabs from shrinking into clipped widths on mobile

Why

Fixes #2106. The install tabs are rendered as a single flex row, but the mobile styles did not provide overflow handling for the long "Build from Source" label.

Validation

• git diff --check
• inspected the generated diff for the install tabs stylesheet

Notes

• I did not run a local Hugo render in this environment because the required hugo extended binary was not installed.