Refine iOS navigation, media controls, and background sync

lib/widgets/selection_bar_overlay.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/material.dart';
+
+/// Pins selection actions above the shell navigation. Scaffold supplies its
+/// navigation footprint through MediaQuery padding when the body extends
+/// behind navigation; otherwise the body already ends above it.
+class SelectionBarOverlay extends StatelessWidget {
+  static const clearance = 64.0;
+
+  final Widget child;
+  final Widget? bar;
+
+  const SelectionBarOverlay({super.key, required this.child, this.bar});
+
+  @override
+  Widget build(BuildContext context) {
+    if (bar == null) return child;
+    return Stack(
+      fit: StackFit.expand,
+      children: [
+        child,
+        Positioned(
+          left: 0,
+          right: 0,
+          bottom: MediaQuery.paddingOf(context).bottom + 8,
+          child: SizedBox(height: 56, child: bar),
+        ),
+      ],
+    );
+  }
+}