fix(web): guard ResizeObserver before constructing it

HAPI Bot Major (PR tiann#741): Vitest jsdom does not polyfill ResizeObserver,
so the toolbar measure effect throws ReferenceError when the existing
mermaid-diagram React tests open the lightbox. Same code path is also
brittle in any browser/webview without the API.

Fall back to plain window 'resize' listener when ResizeObserver is
absent. Toolbar height won't auto-update on element resize without it,
but the lightbox still renders and the resize listener catches the
common viewport-rotation case.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: heavygee

web/src/components/ZoomableLightbox.tsx

@@ -359,9 +359,14 @@ export function ZoomableLightbox(props: ZoomableLightboxProps) {
         }
 
         apply()
+        window.addEventListener('resize', apply)
+
+        if (typeof ResizeObserver === 'undefined') {
+            return () => window.removeEventListener('resize', apply)
+        }
+
         const resize = new ResizeObserver(apply)
         resize.observe(toolbar)
-        window.addEventListener('resize', apply)
         return () => {
             resize.disconnect()
             window.removeEventListener('resize', apply)