allow root relative paths (#1549)

## Launch Checklist

<!-- Thanks for the PR! Feel free to add or remove items from the
checklist. -->


 - [x] Briefly describe the changes in this PR.

when launching maputnik with `--static /data` those files are available
under e.g. `localhost:8000/static`. In this case, root relative urls are
enough (e.g. requsting `/static/fonts`...). This does currently lead to
a validation error:

<img width="361" height="61" alt="grafik"
src="https://github.com/user-attachments/assets/2be1d649-51c9-4e64-ac44-c0089b462be2"
/>

- [x] Include before/after visuals or gifs if this PR includes visual
changes.

after this change it works:

<img width="338" height="68" alt="grafik"
src="https://github.com/user-attachments/assets/27f23d1b-11f6-4efc-b508-2f51bf254953"
/>

 - [x] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Upgraded codemirror from version 5 to version 6
 - Add code editor to allow editing the entire style
 - Add support for sprite object in setting modal
+- Allow root-relative urls in the stylefile
 - _...Add new stuff here..._
 
 ### 🐞 Bug fixes