fix(ui): Ensure spinner box-sizing prevents shift (#9147)

Author: alexcarpenter

.changeset/spinner-box-sizing.md

@@ -0,0 +1,5 @@
+---
+'@clerk/ui': patch
+---
+
+Set `box-sizing: border-box` on the spinner so its border no longer changes the rendered size and causes a layout shift.

packages/ui/src/primitives/Spinner.tsx

@@ -6,6 +6,7 @@ const { size, thickness, speed } = createCssVariables('speed', 'size', 'thicknes
 const { applyVariants, filterProps } = createVariants(theme => {
   return {
     base: {
+      boxSizing: 'border-box',
       display: 'inline-block',
       borderRadius: '99999px',
       borderTop: `${thickness} solid currentColor`,