Refactor button component and add new sections for loading indicators, icons, and navigation; update enums and pubspec description

apps/gallery/lib/sections/section_card.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:m3e_collection/m3e_collection.dart';
+
+class SectionCard extends StatelessWidget {
+  const SectionCard(
+      {super.key, required this.title, this.subtitle, required this.child});
+
+  final String title;
+  final String? subtitle;
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final m3e =
+        theme.extension<M3ETheme>() ?? M3ETheme.defaults(theme.colorScheme);
+
+    return Padding(
+      padding: const EdgeInsets.only(bottom: 24),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          ListTile(
+            contentPadding: EdgeInsets.zero,
+            title: Text(title, style: theme.textTheme.titleLarge),
+            subtitle: subtitle == null ? null : Text(subtitle!),
+          ),
+          Material(
+            color: theme.colorScheme.surfaceContainerLow,
+            borderRadius: m3e.shapes.round.lg,
+            child: Padding(
+              padding: const EdgeInsets.all(16),
+              child: child,
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}