feat(legacy): replace CircularProgressIndicator with animated GIF loading

Add LegacyLoadingIndicator widget using a transparent GIF animation
for all full-page loading states across legacy modules. Also fix
HealthController crash by deferring provider mutation during build.

packages/design_system/lib/design_system.dart

@@ -15,5 +15,6 @@ export 'src/containers/section_container.dart';
 export 'src/containers/footer_container.dart';
 export 'src/rows/editable_row.dart';
 export 'src/loading/app_loading_indicator.dart';
+export 'src/loading/legacy_loading_indicator.dart';
 export 'src/confetti/confetti_overlay.dart';
 export 'src/dialogs/contacts_permission_dialog.dart';

packages/design_system/lib/src/loading/legacy_loading_indicator.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+
+class LegacyLoadingIndicator extends StatelessWidget {
+  final double size;
+
+  const LegacyLoadingIndicator({super.key, this.size = 120});
+
+  @override
+  Widget build(BuildContext context) {
+    return Center(
+      child: Image.asset(
+        'packages/design_system/assets/animations/loading_legacy.gif',
+        width: size,
+        height: size,
+      ),
+    );
+  }
+}