fix: align font size for line numbers with the code editor (#4862)

## 📝 Summary

The font size for the line numbers remained unchanged when the "Code
editor font size" was changed in the user settings. The following video
demonstrates the difference when the font size is increased to 16px
before and after applying the patch.


https://github.com/user-attachments/assets/63bd8ec5-c380-46a3-9527-843b1eefed89

## 🔍 Description of Changes
I added a CSS class that uses the font size for the code editor on the
line numbers.

## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

## 📜 Reviewers

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@akshayka OR @mscolnick

Author: yjoer

frontend/src/css/app/Cell.css

@@ -334,6 +334,10 @@
   font-size: var(--marimo-code-editor-font-size, 0.9rem);
 }
 
+.cm-lineNumbers {
+  font-size: var(--marimo-code-editor-font-size, 0.9rem);
+}
+
 /* .marimo-cell is needed to take precedence over codemirror's generated class ... */
 .marimo-cell .cm-editor {
   border: 1px solid transparent;