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/modules/animations.css

@@ -26,7 +26,7 @@
   border-width: 4px;
   border-style: solid;
   border-color: var(--color-secondary) var(--color-secondary) var(--color-secondary-dark-8) var(--color-secondary-dark-8);
-  border-radius: 100%;
+  border-radius: var(--border-radius-circle);
 }
 
 .is-loading.small-loading-icon::after {