fix: Use scrollToIndex with viewPosition:1 for reliable bottom scroll

- scrollToIndex targets last message at bottom of viewport
- onScrollToIndexFailed fallback to scrollToEnd
- More reliable than scrollToEnd with dynamic heights

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

android/src/screens/ChatScreen.tsx

@@ -377,12 +377,21 @@ const ChatScreen: React.FC = () => {
   useEffect(() => {
     if (messages.length > 0 && shouldAutoScroll.current) {
       const isInitial = lastMessageCount.current === 0;
-      // Mehrfach versuchen (FlatList braucht Zeit zum Rendern)
-      const delays = isInitial ? [100, 300, 600, 1000] : [100, 300];
-      for (const delay of delays) {
-        setTimeout(() => {
+      const scrollToBottom = () => {
+        try {
+          flatListRef.current?.scrollToIndex({
+            index: messages.length - 1,
+            animated: !isInitial,
+            viewPosition: 1, // 1 = Item am unteren Rand
+          });
+        } catch {
+          // Fallback wenn Index nicht berechnet werden kann
           flatListRef.current?.scrollToEnd({ animated: !isInitial });
-        }, delay);
+        }
+      };
+      const delays = isInitial ? [200, 500, 1000] : [150];
+      for (const delay of delays) {
+        setTimeout(scrollToBottom, delay);
       }
     }
     lastMessageCount.current = messages.length;
@@ -691,6 +700,11 @@ const ChatScreen: React.FC = () => {
         showsVerticalScrollIndicator={false}
         onScrollBeginDrag={handleScrollBeginDrag}
         onScrollEndDrag={handleScrollEndDrag}
+        onScrollToIndexFailed={(info) => {
+          setTimeout(() => {
+            flatListRef.current?.scrollToEnd({ animated: false });
+          }, 200);
+        }}
         ListEmptyComponent={
           <View style={styles.emptyContainer}>
             <Text style={styles.emptyIcon}>{'\uD83E\uDD16'}</Text>