implemented basic layout & functionality for add group button & group tiles

lib/presentation/widgets/full_width_button.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+import 'package:game_tracker/core/custom_theme.dart';
+
+class FullWidthButton extends StatelessWidget {
+  const FullWidthButton({super.key, required this.text, this.onPressed});
+
+  final String text;
+  final VoidCallback? onPressed;
+
+  @override
+  Widget build(BuildContext context) {
+    return ElevatedButton(
+      onPressed: onPressed,
+      style: ElevatedButton.styleFrom(
+        minimumSize: Size(MediaQuery.of(context).size.width * 0.8, 60),
+        backgroundColor: CustomTheme.primaryColor,
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
+      ),
+      child: Text(
+        text,
+        style: const TextStyle(
+          fontWeight: FontWeight.w500,
+          fontSize: 22,
+          color: Colors.white,
+        ),
+      ),
+    );
+  }
+}