feat(message): add pinned conversations sorting and unified timeline

Sort conversation list with pinned items first, then by updated_at timestamp.
Also fixes timestamp text alignment by adding lineHeight, textAlignVertical,
and includeFontPadding properties for consistent vertical centering.

src/screens/message/MessageListScreen.tsx

@@ -475,11 +475,16 @@ export const MessageListScreen: React.FC = () => {
     };
   };
 
-  // 合并列表数据
+  // 合并列表数据：系统消息与普通会话一起按时间排序
   const listData: ConversationResponse[] = [
     createSystemMessageItem(),
     ...conversationList,
-  ];
+  ].sort((a, b) => {
+    const aPinned = a.is_pinned ? 1 : 0;
+    const bPinned = b.is_pinned ? 1 : 0;
+    if (aPinned !== bPinned) return bPinned - aPinned;
+    return new Date(b.updated_at).getTime() - new Date(a.updated_at).getTime();
+  });
 
   const listConvRef = useRef(new Map<string, ConversationResponse>());
   listConvRef.current = new Map(listData.map((c) => [String(c.id), c]));
@@ -981,6 +986,9 @@ function createMessageListStyles(colors: AppColors) {
       color: colors.primary.contrast,
       fontSize: 11,
       fontWeight: '600',
+      lineHeight: 18,
+      textAlignVertical: 'center',
+      includeFontPadding: false,
     },
     headerRight: {
       width: 44,

src/screens/message/components/ConversationListRow.tsx

@@ -265,6 +265,9 @@ function createConversationRowStyles(colors: AppColors) {
       color: '#FFFFFF',
       fontSize: 12,
       fontWeight: '700',
+      lineHeight: 20,
+      textAlignVertical: 'center',
+      includeFontPadding: false,
     },
   });
 }