fix: Auto-scroll to last message on app start + new messages

- useEffect on messages array instead of onContentSizeChange
- Instant jump (no animation) when loading history
- Animated scroll for single new messages
- Scroll pauses when user scrolls up, resumes at bottom

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/src/screens/ChatScreen.tsx

@@ -361,18 +361,25 @@ const ChatScreen: React.FC = () => {
     return () => { if (saveTimer.current) clearTimeout(saveTimer.current); };
   }, [messages]);
 
-  // Auto-Scroll wird ueber onContentSizeChange der FlatList gesteuert
+  // Auto-Scroll: immer zur letzten Nachricht springen
   const shouldAutoScroll = useRef(true);
-  const handleContentSizeChange = useCallback(() => {
-    if (shouldAutoScroll.current) {
-      flatListRef.current?.scrollToEnd({ animated: false });
+  const lastMessageCount = useRef(0);
+
+  // Bei neuen Nachrichten: sofort nach unten
+  useEffect(() => {
+    if (messages.length > lastMessageCount.current && shouldAutoScroll.current) {
+      // Kurzer Delay damit FlatList rendern kann
+      setTimeout(() => {
+        flatListRef.current?.scrollToEnd({ animated: messages.length > lastMessageCount.current + 1 ? false : true });
+      }, 150);
     }
-  }, []);
+    lastMessageCount.current = messages.length;
+  }, [messages]);
+
   const handleScrollBeginDrag = useCallback(() => {
     shouldAutoScroll.current = false;
   }, []);
   const handleScrollEndDrag = useCallback((e: any) => {
-    // Auto-Scroll wieder aktivieren wenn User ganz unten ist
     const { contentOffset, contentSize, layoutMeasurement } = e.nativeEvent;
     const isAtBottom = contentOffset.y + layoutMeasurement.height >= contentSize.height - 50;
     shouldAutoScroll.current = isAtBottom;
@@ -671,7 +678,6 @@ const ChatScreen: React.FC = () => {
         renderItem={renderMessage}
         contentContainerStyle={styles.messageList}
         showsVerticalScrollIndicator={false}
-        onContentSizeChange={handleContentSizeChange}
         onScrollBeginDrag={handleScrollBeginDrag}
         onScrollEndDrag={handleScrollEndDrag}
         ListEmptyComponent={