base: Center tab and toggle content (#2839)

## Description

Align `Tab` and `Toggle` with `Button` neutral control geometry by
applying flex centering and a unit line height at the component root.
Fixed-height controls now vertically center ordinary child content while
applications continue to own size, padding, colors, and selected/pressed
styling.

## Screenshot

Not included; this is a base layout correction covered by geometry
regression tests.

## How to Test

```bash
cargo test -p gpui-base fixed_height_tab_centers_ordinary_child_geometry
cargo test -p gpui-base fixed_height_toggle_centers_ordinary_child_geometry
cargo test -p gpui-base --lib
```

Verified on Linux: 572 gpui-base library tests passed.

## Checklist

- [x] I have read the CONTRIBUTING document and followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code is
accurate.
- [x] Passed the story-related component regression tests.
- [x] Tested Linux; the change is platform-independent layout behavior.