feat(material): adopt expressive controls across the app

lib/widgets/app_loading_indicator.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+import 'package:material_3_expressive/material_3_expressive.dart';
+import 'package:spotiflac_android/theme/material_expressive.dart';
+
+/// Indeterminate loading only. Downloads/analysis keep their real progress.
+class AppLoadingIndicator extends StatelessWidget {
+  const AppLoadingIndicator({
+    super.key,
+    this.size = 32,
+    this.color,
+    this.semanticLabel,
+  });
+
+  final double size;
+  final Color? color;
+  final String? semanticLabel;
+
+  @override
+  Widget build(BuildContext context) => SizedBox.square(
+    dimension: size,
+    child: materialExpressiveEnabled(context)
+        ? MaterialExpressiveScope(
+            child: FittedBox(
+              child: M3ELoadingIndicator(
+                size: 48,
+                color: color,
+                semanticLabel: semanticLabel,
+              ),
+            ),
+          )
+        : CircularProgressIndicator(
+            color: color,
+            strokeWidth: 3,
+            semanticsLabel: semanticLabel,
+          ),
+  );
+}