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/helpers/chat_scroll_controller.dart

@@ -0,0 +1,68 @@
+import 'package:flutter/material.dart';
+
+class ChatScrollController extends ScrollController {
+  final ValueNotifier<bool> showJumpToBottom = ValueNotifier(false);
+  VoidCallback? onScrollNearTop;
+
+  static const _bottomThreshold = 100.0;
+  static const _topThreshold = 50.0;
+
+  ChatScrollController() {
+    addListener(_handleScroll);
+  }
+
+  void _handleScroll() {
+    if (!hasClients) return;
+    final pos = position;
+
+    // With reverse: true, position 0 is bottom, maxScrollExtent is top
+    // Show jump button when scrolled away from bottom (position > threshold)
+    final isAtBottom = pos.pixels <= _bottomThreshold;
+    if (showJumpToBottom.value == isAtBottom) {
+      showJumpToBottom.value = !isAtBottom;
+    }
+
+    // Pagination trigger when scrolled near top (maxScrollExtent)
+    if (pos.pixels >= pos.maxScrollExtent - _topThreshold) {
+      onScrollNearTop?.call();
+    }
+  }
+
+  void jumpToBottom() {
+    if (hasClients && position.maxScrollExtent > 0) {
+      animateTo(
+        0, // With reverse: true, position 0 is bottom
+        duration: const Duration(milliseconds: 300),
+        curve: Curves.easeOut,
+      );
+    }
+  }
+
+  void handleKeyboardOpen() {
+    // Simple: just scroll to bottom when keyboard opens
+    if (hasClients) {
+      animateTo(
+        0, // With reverse: true, position 0 is bottom
+        duration: const Duration(milliseconds: 200),
+        curve: Curves.easeOut,
+      );
+    }
+  }
+
+  void scrollToBottomIfAtBottom() {
+    // Only scroll if jump button is NOT showing (i.e., already at bottom)
+    if (!showJumpToBottom.value && hasClients && position.maxScrollExtent > 0) {
+      animateTo(
+        0, // With reverse: true, position 0 is bottom
+        duration: const Duration(milliseconds: 200),
+        curve: Curves.easeOut,
+      );
+    }
+  }
+
+  @override
+  void dispose() {
+    showJumpToBottom.dispose();
+    super.dispose();
+  }
+}