fix: add estimatedItemSize to FlashList to fix black screen issue

FlashList requires estimatedItemSize prop to properly render content. Without it, the list may appear blank even when data is present.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/(tabs)/index.tsx

@@ -393,6 +393,7 @@ export default function HomeScreen() {
                             )}
                             keyExtractor={(item) => item.id!}
                             numColumns={numColumns}
+                            estimatedItemSize={cardWidth * 1.5}
                             showsVerticalScrollIndicator={false}
                             scrollEventThrottle={Platform.OS === 'ios' ? 16 : 50}
                         />