Fix a11y root element having 0×0 dimensions on web

The `cmp_a11y_root` container was created with `position: absolute`
but never given width or height, making it invisible to hit-test-based
accessibility tools (Accessibility Inspector, Appium). VoiceOver still
worked because it traverses the DOM tree sequentially.

Sync the a11y container's CSS dimensions with the canvas in `resize()`,
which runs on both initial layout and every subsequent resize.

Fixes https://youtrack.jetbrains.com/issue/CMP-10172

Assisted-by: cursor

Author: spookyvert

compose/ui/ui/src/webMain/kotlin/androidx/compose/ui/window/ComposeWindowInternal.web.kt

@@ -501,6 +501,11 @@ internal class ComposeWindow(
         canvas.style.width = "${boxSize.width.value}px"
         canvas.style.height = "${boxSize.height.value}px"
 
+        a11yContainerElement?.let {
+            it.style.width = "${boxSize.width.value}px"
+            it.style.height = "${boxSize.height.value}px"
+        }
+
         _windowInfo.containerSize = sizeInPx
         _windowInfo.containerDpSize = boxSize