feat: 通知中心只显示未读消息，标为已读后自动消失

- WorkStationPage: list 请求加 unread_only=true，仅拉取未读通知
- RecommendationDrawer: 移除已读/未读条件渲染，列表项统一以未读样式展示
- 标为已读 / 全部已读后触发 onRefresh，已读条目从列表消失
- 空列表提示改为"暂无未读通知"
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/workstation/RecommendationDrawer.tsx

@@ -40,25 +40,20 @@ export function RecommendationDrawer({ open, notifications, onClose, onRefresh }
 
       <List sx={{ px: 2, overflow: 'auto' }}>
         {notifications.length === 0 && (
-          <Typography variant="body2" color="text.secondary" sx={{ p: 2 }}>暂无通知</Typography>
+          <Typography variant="body2" color="text.secondary" sx={{ p: 2 }}>暂无未读通知</Typography>
         )}
         {notifications.map(n => (
           <ListItem
             key={n.id}
             divider
             alignItems="flex-start"
-            sx={{
-              bgcolor: n.is_read ? 'transparent' : '#e3f2fd',
-              borderRadius: 1,
-              mb: 0.5,
-              px: 1,
-            }}
+            sx={{ bgcolor: '#e3f2fd', borderRadius: 1, mb: 0.5, px: 1 }}
           >
             <ListItemText
               primary={
                 <Stack direction="row" spacing={1} alignItems="center">
-                  <Typography variant="body2" fontWeight={n.is_read ? 400 : 600}>{n.title}</Typography>
-                  {!n.is_read && <Chip label="新" size="small" color="primary" />}
+                  <Typography variant="body2" fontWeight={600}>{n.title}</Typography>
+                  <Chip label="新" size="small" color="primary" />
                 </Stack>
               }
               secondary={
@@ -70,11 +65,9 @@ export function RecommendationDrawer({ open, notifications, onClose, onRefresh }
                 </>
               }
             />
-            {!n.is_read && (
             <IconButton size="small" onClick={() => handleMarkRead(n.id)} title="标为已读">
               <CheckCircleIcon fontSize="small" color="primary" />
             </IconButton>
-            )}
           </ListItem>
         ))}
       </List>

frontend/src/pages/WorkStationPage.tsx

@@ -113,13 +113,13 @@ export function WorkStationPage() {
 
   const loadNotifications = useCallback(async () => {
     const [notifs, count] = await Promise.all([
-      notificationService.list({ limit: 50 }),
+      notificationService.list({ unread_only: true, limit: 50 }),
       notificationService.unreadCount(),
     ])
     setNotifications(notifs)
     setUnreadCount(count.count)
 
-    const latestUnread = notifs.find(n => !n.is_read)
+    const latestUnread = notifs[0]
     if (latestUnread && !shownNotificationIds.current.has(latestUnread.id)) {
       shownNotificationIds.current.add(latestUnread.id)
       setSnackNotification(latestUnread)