feat: add ChatScrollController and JumpToBottomButton for improved chat scrolling experience

- Implemented ChatScrollController to manage scroll behavior and visibility of jump-to-bottom button.
- Added functionality to automatically scroll to the bottom when the keyboard opens.
- Created JumpToBottomButton widget that appears when the user scrolls up, allowing quick navigation back to the bottom of the chat.

lib/widgets/jump_to_bottom_button.dart

@@ -0,0 +1,29 @@
+import 'package:flutter/material.dart';
+import '../helpers/chat_scroll_controller.dart';
+
+class JumpToBottomButton extends StatelessWidget {
+  final ChatScrollController scrollController;
+
+  const JumpToBottomButton({
+    super.key,
+    required this.scrollController,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder<bool>(
+      valueListenable: scrollController.showJumpToBottom,
+      builder: (context, show, _) {
+        if (!show) return const SizedBox.shrink();
+        return Positioned(
+          right: 16,
+          bottom: 16,
+          child: FloatingActionButton.small(
+            onPressed: scrollController.jumpToBottom,
+            child: const Icon(Icons.keyboard_arrow_down),
+          ),
+        );
+      },
+    );
+  }
+}