Change --border-radius-circle to --border-radius-full (gitea#30936)

Cherry-pick of 2ced31e81d adapted to Forgejo releases UI.

Percentage-based `border-radius` [creates undesirable
ellipse](https://jsfiddle.net/silverwind/j9ko5wnt/4/) on non-square
content. Instead, use pixel value and use same wording `full` like
tailwind does, but increast to 99999px over their 9999px.

(cherry picked from commit 2ced31e81dd9e45659660c1abff529d0192fd8ed)

tailwind.config.js

@@ -66,7 +66,7 @@ export default {
       'xl': '12px',
       '2xl': '16px',
       '3xl': '24px',
-      'full': 'var(--border-radius-circle)', // 50%
+      'full': 'var(--border-radius-full)',
     },
     fontFamily: {
       sans: 'var(--fonts-regular)',