fix: restore copy icon visibility on code block hover

The SVG path fill was using `hsla(var(--foreground-secondary))` which
references a CSS variable that doesn't exist in Tailwind v4 — the correct
variable is `--color-foreground-secondary`. This made the copy icon
invisible while the button itself was still present and clickable.

Changed the fill to `currentColor` so the icon inherits the correct color
from the button's `text-foreground-secondary` CSS class.

Author: donjo

markdown-it/codeblock-copy.ts

@@ -22,7 +22,7 @@ export default function codeblockCopyPlugin(md: any) {
         <svg class="copy-icon" width="15" height="15" viewBox="0 0 15 15" fill="none" xmlns="http://www.w3.org/2000/svg">
           <rect x="2" y="2" width="7" height="7" fill="none"/>
           <rect x="6" y="6" width="7" height="7" fill="none"/>
-          <path d="M1.55566 2.7C1.55566 2.03726 2.09292 1.5 2.75566 1.5H8.75566C9.41841 1.5 9.95566 2.03726 9.95566 2.7V5.1H12.3557C13.0184 5.1 13.5557 5.63726 13.5557 6.3V12.3C13.5557 12.9627 13.0184 13.5 12.3557 13.5H6.35566C5.69292 13.5 5.15566 12.9627 5.15566 12.3V9.9H2.75566C2.09292 9.9 1.55566 9.36274 1.55566 8.7V2.7ZM6.35566 9.9V12.3H12.3557V6.3H9.95566V8.7C9.95566 9.36274 9.41841 9.9 8.75566 9.9H6.35566ZM8.75566 8.7V2.7H2.75566V8.7H8.75566Z" fill="hsla(var(--foreground-secondary))"/>
+          <path d="M1.55566 2.7C1.55566 2.03726 2.09292 1.5 2.75566 1.5H8.75566C9.41841 1.5 9.95566 2.03726 9.95566 2.7V5.1H12.3557C13.0184 5.1 13.5557 5.63726 13.5557 6.3V12.3C13.5557 12.9627 13.0184 13.5 12.3557 13.5H6.35566C5.69292 13.5 5.15566 12.9627 5.15566 12.3V9.9H2.75566C2.09292 9.9 1.55566 9.36274 1.55566 8.7V2.7ZM6.35566 9.9V12.3H12.3557V6.3H9.95566V8.7C9.95566 9.36274 9.41841 9.9 8.75566 9.9H6.35566ZM8.75566 8.7V2.7H2.75566V8.7H8.75566Z" fill="currentColor"/>
         </svg>
         <svg class="check-icon hidden" width="15" height="15" viewBox="0 0 15 15" fill="none" xmlns="http://www.w3.org/2000/svg">
           <path d="M3 7.5L6.5 11L12 3" stroke="green" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>