fix: show LoadingOverlay during CDN metadata fetch, not just template load

Root cause: the LoadingOverlay lives inside WebIframe, which only mounts
after defaultWebPreviewFile is set (i.e. after metadata finishes fetching).
During the CDN metadata fetch window there is no WebIframe in the tree at
all, so no loading indicator is visible. Additionally, once the CDN bundle
is browser-cached the template renders in ~2 rAFs which is imperceptible.

Fix: import LoadingOverlay into ExampleContent and render it directly on
the preview-wrap-content panel with visible={!initState}. This covers the
entire metadata-fetch phase. The WebIframe's own overlay continues to
handle the subsequent template-load phase, giving a seamless indicator
from the moment the CDN version is requested until the frame is painted.

Author: Huxpro

src/example-preview/components/index.tsx

@@ -26,6 +26,7 @@ import { FileTree } from './file-tree';
 import { CodeView } from './code-view';
 import { SwitchSchema } from './switch-schema';
 import { PreviewImg } from './preview-img';
+import { LoadingOverlay } from './loading-overlay';
 import { SplitPane, type SplitPaneHandle } from './split-pane';
 
 import {
@@ -278,7 +279,7 @@ export const ExampleContent: FC<ExampleContentProps> = ({
               }
               second={
                 <div className={s['preview-wrap']}>
-                  <div className={s['preview-wrap-content']}>
+                  <div className={s['preview-wrap-content']} style={{ position: 'relative' }}>
                     <div className={s['preview-header']}>
                       <div style={{ width: 24, flexShrink: 0 }} />
                       <RadioGroup
@@ -342,6 +343,9 @@ export const ExampleContent: FC<ExampleContentProps> = ({
                       />
                     </div>
 
+                    {/* Show loading overlay while metadata is fetching (before initState=true) */}
+                    <LoadingOverlay visible={!initState} />
+
                     {previewType === PreviewType.QRCode && currentEntry && (
                       <div className={s.qrcode}>
                         <Typography.Text