feat(electron): electron shell skeleton (#8127)

fix AF-1331
<div class='graphite__hidden'>
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<video src="https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/T2klNLEk0wxLh4NRDzhk/e09203aa-f143-42f8-bd39-e5078d07ada2.mp4">1.mp4</video>

missing
- per split view skeleton
- per route skeleton

packages/frontend/electron/renderer/shell/shell.tsx

@@ -1,5 +1,7 @@
+import { AppFallback } from '@affine/core/components/affine/app-container';
 import { useAppSettingHelper } from '@affine/core/hooks/affine/use-app-setting-helper';
 import { AppTabsHeader } from '@affine/core/modules/app-tabs-header';
+import { SplitViewFallback } from '@affine/core/modules/workbench/view/split-view/split-view';
 
 import * as styles from './shell.css';
 
@@ -11,7 +13,10 @@ export function ShellRoot() {
     appSettings.enableBlurBackground;
   return (
     <div className={styles.root} data-translucent={translucent}>
-      <AppTabsHeader mode="shell" />
+      <AppTabsHeader mode="shell" className={styles.appTabsHeader} />
+      <AppFallback className={styles.fallbackRoot}>
+        <SplitViewFallback className={styles.splitViewFallback} />
+      </AppFallback>
     </div>
   );
 }