fix: Correct chat scroll behavior for both sort modes

Add scrollToLatestMessage helper that respects sortNewestFirst setting:
- When newest first: scroll to top (offset 0)
- When oldest first: scroll to end

Applied to keyboard show, voice messages, and content size changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/(tabs)/chat.tsx

@@ -476,18 +476,29 @@ export default function ChatScreen() {
     autoSelect();
   }, []);
 
-  // Scroll to end when keyboard shows
+  // Helper function to scroll to the latest message based on sort mode
+  const scrollToLatestMessage = useCallback((animated = true) => {
+    if (sortNewestFirst) {
+      // When newest first, latest messages are at top (index 0)
+      flatListRef.current?.scrollToOffset({ offset: 0, animated });
+    } else {
+      // When oldest first, latest messages are at bottom
+      flatListRef.current?.scrollToEnd({ animated });
+    }
+  }, [sortNewestFirst]);
+
+  // Scroll to latest when keyboard shows
   useEffect(() => {
     const keyboardShowListener = Keyboard.addListener(
       Platform.OS === 'ios' ? 'keyboardWillShow' : 'keyboardDidShow',
       () => {
         setTimeout(() => {
-          flatListRef.current?.scrollToEnd({ animated: true });
+          scrollToLatestMessage(true);
         }, 100);
       }
     );
     return () => keyboardShowListener.remove();
-  }, []);
+  }, [scrollToLatestMessage]);
 
   const openBeneficiaryPicker = useCallback(() => {
     setShowBeneficiaryPicker(true);
@@ -588,14 +599,14 @@ export default function ChatScreen() {
 
     setMessages(prev => [...prev, voiceMessage]);
 
-    // Scroll to bottom
+    // Scroll to latest message (respects sort mode)
     setTimeout(() => {
-      flatListRef.current?.scrollToEnd({ animated: true });
+      scrollToLatestMessage(true);
     }, 100);
 
     // Also store in transcript context for persistence
     addTranscriptEntry(role, text);
-  }, [hasShownVoiceSeparator, addTranscriptEntry]);
+  }, [hasShownVoiceSeparator, addTranscriptEntry, scrollToLatestMessage]);
 
   // Cached API token for WellNuo
   const apiTokenRef = useRef<string | null>(null);
@@ -887,9 +898,7 @@ export default function ChatScreen() {
           contentContainerStyle={styles.messagesList}
           showsVerticalScrollIndicator={false}
           onContentSizeChange={() => {
-            if (!sortNewestFirst) {
-              flatListRef.current?.scrollToEnd({ animated: true });
-            }
+            scrollToLatestMessage(true);
           }}
         />