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/repo/issue-list.css

@@ -39,7 +39,7 @@
 
 #issue-list .flex-item-body .branches .branch {
   background-color: var(--color-secondary-alpha-40);
-  border-radius: 3px;
+  border-radius: var(--border-radius);
   padding: 0 4px;
 }
 
@@ -55,7 +55,6 @@
   width: 80px;
   height: 6px;
   display: inline-block;
-  border-radius: 3px;
 }
 
 #issue-list .flex-item-body .checklist progress::-webkit-progress-value {
@@ -64,4 +63,4 @@
 
 #issue-list .flex-item-body .checklist progress::-moz-progress-bar {
   background-color: var(--color-secondary-dark-4);
-}
+}