have back button dismiss lightbox on web (#9874)

src/alf/atoms.ts

@@ -108,6 +108,22 @@ export const atoms = {
     animation: `zoomIn ${EXP_CURVE} 0.3s, fadeIn ${EXP_CURVE} 0.3s`,
   }),
 
+  /**
+   * Visually hidden but available to screen readers (web).
+   * Use for live regions or off-screen labels (e.g. "Image 1 of 3").
+   */
+  sr_only: web({
+    position: 'absolute',
+    width: 1,
+    height: 1,
+    padding: 0,
+    margin: -1,
+    overflow: 'hidden',
+    clip: 'rect(0,0,0,0)',
+    whiteSpace: 'nowrap',
+    borderWidth: 0,
+  }),
+
   /**
    * {@link Layout.SCROLLBAR_OFFSET}
    */