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

apps/gallery/lib/sections/split_button_section.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+import 'package:m3e_collection/m3e_collection.dart';
+
+import 'section_card.dart';
+
+class SplitButtonSection extends StatelessWidget {
+  const SplitButtonSection({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final items = const [
+      SplitButtonM3EItem<String>(value: 'one', child: 'One'),
+      SplitButtonM3EItem<String>(value: 'two', child: 'Two'),
+      SplitButtonM3EItem<String>(value: 'three', child: 'Three'),
+    ];
+
+    return SectionCard(
+      title: 'SplitButtonM3E',
+      subtitle: 'Generated from enums: emphasis × size (round shape).',
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          for (final emphasis in SplitButtonM3EEmphasis.values) ...[
+            Padding(
+              padding: const EdgeInsets.symmetric(vertical: 8),
+              child: Text(emphasis.name, style: Theme.of(context).textTheme.titleMedium),
+            ),
+            Wrap(
+              spacing: 12,
+              runSpacing: 12,
+              children: [
+                for (final size in SplitButtonM3ESize.values)
+                  SplitButtonM3E<String>(
+                    label: emphasis.name,
+                    size: size,
+                    emphasis: emphasis,
+                    onPressed: () {},
+                    items: items,
+                  ),
+              ],
+            ),
+          ],
+        ],
+      ),
+    );
+  }
+}