feat(chat): implement @mention chip overlay rendering with Unicode placeholder tracking

Add visual overlay rendering for @mention chips in the chat input using Unicode private use area characters as invisible placeholders. The implementation includes chip data tracking via `MentionChipMap`, dynamic segment parsing to separate chip and text content for overlay display, and updated memo comparison in `MessageBubble` to include avatar press handlers for proper re-render prevention.

src/screens/message/components/ChatScreen/styles.ts

@@ -556,6 +556,29 @@ export function createChatScreenStyles(colors: AppColors, dynamicStyles?: {
   inputTransparent: {
     color: 'transparent',
   },
+  mentionOverlay: {
+    ...StyleSheet.absoluteFillObject,
+    flexDirection: 'row',
+    flexWrap: 'wrap',
+    alignItems: 'flex-start',
+    paddingTop: 8,
+    paddingBottom: 8,
+    paddingHorizontal: spacing.xs,
+  },
+  mentionChipText: {
+    fontSize: 16,
+    color: colors.primary.main,
+    fontWeight: '500',
+    lineHeight: 20,
+    backgroundColor: `${colors.primary.main}15`,
+    borderRadius: 4,
+    overflow: 'hidden',
+  },
+  mentionOverlayText: {
+    fontSize: 16,
+    color: colors.chat.textPrimary,
+    lineHeight: 20,
+  },
   
   // 输入框@高亮
   inputHighlightOverlay: {