feat(apple): cover the window while the core boots

The core initializes asynchronously at launch, so for a moment the transfer lists
look empty and the app feels stalled. Show a full-window overlay (centered spinner
+ "Starting…") as the top layer of the root stack while coreState.isInitialized is
false; it fades out once the core is ready.

shared/src/commonMain/composeResources/values-nl/strings.xml

@@ -22,6 +22,7 @@
     <string name="about_privacy_title">Privacy en beveiliging</string>
     <string name="about_tagline">Verstuur bestanden rechtstreeks. Houd controle over wie ze ontvangt.</string>
     <string name="about_title">Over</string>
+    <string name="app_starting">Bezig met starten…</string>
     <string name="appearance_auto_description">De lichte of donkere weergave van dit apparaat volgen.</string>
     <string name="appearance_dark_mode">Donkere modus</string>
     <string name="appearance_light_mode">Lichte modus</string>