base: Fix to default center button content (#2835)

## Summary

- establish a flex formatting context on the base Button root
- center arbitrary button children on both axes
- add a real layout regression test for fixed-height buttons

A relative 1.0 line height keeps the label line box neutral; flex
centering owns the control geometry. This fixes Button-backed tabs and
buttons without story-specific padding.

## Verification

- cargo test -p gpui-base --lib (570 passed)

Tests were run directly without proxy bypass environment variables.