Reduce the height of the the tab bars and side panel titles (#5609)

### What

Small step towards a more efficient use of vertical space. Set to 24px,
down from 28px. Now on par with `ListItem`:

<img width="371" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/85ec7b25-b547-4a39-87d1-c4d6c1427d3d">


<img width="1535" alt="image"
src="https://github.com/rerun-io/rerun/assets/49431240/7146f6c8-b1bd-4893-98b4-8d2050b7a45d">


- Part of #5589


### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5609/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5609/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5609/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5609)
- [Docs
preview](https://rerun.io/preview/24c1f9ff1fd6d36c6ae46901a2779b4ab105ab66/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/24c1f9ff1fd6d36c6ae46901a2779b4ab105ab66/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_ui/src/lib.rs

@@ -184,7 +184,7 @@ impl ReUi {
     /// Height of the title row in the blueprint view and selection view,
     /// as well as the tab bar height in the viewport view.
     pub fn title_bar_height() -> f32 {
-        28.0 // from figma 2022-02-03
+        24.0 // https://github.com/rerun-io/rerun/issues/5589
     }
 
     pub fn list_item_height() -> f32 {