fix(flutter): strip duplicate status-bar inset when update banner shows

You correctly diagnosed the symptom: the AppBar gets taller when the
banner is present. Why: the shell is [Banner, Expanded(child),
PlayerBar] and the banner uses SafeArea(bottom:false) to clear the
status bar. But MediaQuery.padding is screen-relative — the child's
Scaffold/AppBar reads MediaQuery.padding.top and applies its own
status-bar inset on top of the banner, doubling the gap.

Watch shouldShowUpdateBannerProvider in the shell. When it returns
non-null (banner visible), wrap the child in MediaQuery.removePadding
removeTop: true so the AppBar mounts directly under the banner. When
the banner is hidden, child gets its normal top inset.

flutter_client/lib/shared/routing.dart

@@ -18,6 +18,7 @@ import '../playlists/playlists_list_screen.dart';
 import '../requests/requests_screen.dart';
 import '../search/search_screen.dart';
 import '../settings/settings_screen.dart';
+import '../update/client_update_provider.dart';
 import '../update/update_banner.dart';
 import '../admin/admin_landing_screen.dart';
 import '../admin/admin_requests_screen.dart';
@@ -88,15 +89,30 @@ GoRouter buildRouter(Ref ref) {
   );
 }
 
-class _ShellWithPlayerBar extends StatelessWidget {
+class _ShellWithPlayerBar extends ConsumerWidget {
   const _ShellWithPlayerBar({required this.child});
   final Widget child;
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
+    // The banner sits above the routed child and uses SafeArea to clear
+    // the system status bar. MediaQuery.padding is screen-relative
+    // though, so the child's Scaffold/AppBar would still apply its own
+    // top status-bar inset on top of the banner — doubling the gap.
+    // When the banner is showing, strip that inset from the child so
+    // its AppBar lands directly under the banner.
+    final hasBanner = ref.watch(shouldShowUpdateBannerProvider) != null;
     return Column(
       children: [
         const UpdateBanner(),
-        Expanded(child: child),
+        Expanded(
+          child: hasBanner
+              ? MediaQuery.removePadding(
+                  context: context,
+                  removeTop: true,
+                  child: child,
+                )
+              : child,
+        ),
         const PlayerBar(),
       ],
     );