refactor(chat): remove Unicode placeholder-based mention chip overlay system

Replaced the previous approach of tracking @mentions using private Unicode characters (U+E000-U+F8FF) with a simpler regex-based solution that extracts mention ranges directly from input text. This eliminates the complex overlay rendering logic and corresponding styles/types.

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

@@ -553,33 +553,7 @@ export function createChatScreenStyles(colors: AppColors, dynamicStyles?: {
   inputMuted: {
     color: colors.chat.iconMuted,
   },
-  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: {
     ...StyleSheet.absoluteFillObject,