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;
 }

web_src/css/features/console.css

@@ -4,7 +4,7 @@
   background: var(--color-console-bg);
   color: var(--color-console-fg);
   font-family: var(--fonts-monospace);
-  border-radius: 5px;
+  border-radius: var(--border-radius);
   word-break: break-word;
   overflow-wrap: break-word;
 }

web_src/css/features/dropzone.css

@@ -3,7 +3,7 @@
   background: none;
   box-shadow: none;
   padding: 0;
-  border-radius: 4px;
+  border-radius: var(--border-radius-medium);
   min-height: 0;
 }
 

web_src/css/features/tribute.css

@@ -2,7 +2,7 @@
 
 .tribute-container {
   box-shadow: 0 0.25rem 0.5rem rgba(0, 0, 0, 0.25);
-  border-radius: 0.25rem;
+  border-radius: var(--border-radius);
 }
 
 .tribute-container ul {