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

apps/gallery/lib/sections/button_section.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+import 'package:m3e_collection/m3e_collection.dart';
+
+import 'section_card.dart';
+
+class ButtonSection extends StatelessWidget {
+  const ButtonSection({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return SectionCard(
+      title: 'ButtonM3E',
+      subtitle:
+          'Generated from enums: variant × size; grouped by shape family.',
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          for (final shape in ButtonM3EShape.values) ...[
+            Padding(
+              padding: const EdgeInsets.symmetric(vertical: 8),
+              child: Text('${shape.name} shape',
+                  style: Theme.of(context).textTheme.titleMedium),
+            ),
+            for (final size in ButtonM3ESize.values) ...[
+              Padding(
+                padding: const EdgeInsets.only(top: 8, bottom: 4),
+                child: Text('size: ${size.name}',
+                    style: Theme.of(context).textTheme.labelLarge),
+              ),
+              Wrap(
+                spacing: 12,
+                runSpacing: 12,
+                children: [
+                  for (final variant in ButtonM3EStyle.values)
+                    ButtonM3E(
+                      label: Text(variant.name),
+                      style: variant,
+                      size: size,
+                      shape: shape,
+                      onPressed: () {},
+                    ),
+                ],
+              ),
+            ],
+          ],
+        ],
+      ),
+    );
+  }
+}