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/markup/content.css

@@ -180,7 +180,7 @@
   appearance: none;
   position: relative;
   border: 1px solid var(--color-secondary);
-  border-radius: 2px;
+  border-radius: var(--border-radius);
   background: var(--color-input-background);
   height: 14px;
   width: 14px;
@@ -433,7 +433,7 @@
   font-size: 85%;
   white-space: break-spaces;
   background-color: var(--color-markup-code-block);
-  border-radius: 4px;
+  border-radius: var(--border-radius);
 }
 
 .markup code br,
@@ -466,7 +466,7 @@
   font-size: 85%;
   line-height: 1.45;
   background-color: var(--color-markup-code-block);
-  border-radius: 4px;
+  border-radius: var(--border-radius);
 }
 
 .markup .highlight pre {
@@ -504,7 +504,7 @@
   vertical-align: middle;
   background-color: var(--color-markup-code-block);
   border: 1px solid var(--color-secondary);
-  border-radius: 3px;
+  border-radius: var(--border-radius);
   box-shadow: inset 0 -1px 0 var(--color-secondary);
 }