Unify border-radius behavior (#26770)

## Changes
- no more hardcoded `border-radius`es (apart from `0`)
- no more value inconsistencies
- no more guessing what pixel value you should use
- two new variables:
- `--border-radius-medium` (for elements where the normal border radius
does not suffice)
  - `--border-radius-circle` (for displaying circles)

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/features/codeeditor.css

@@ -44,5 +44,5 @@
   position: absolute !important;
   resize: none !important;
   overflow: hidden !important;
-  border-radius: 4px !important;
+  border-radius: var(--border-radius-medium) !important;
 }