fix(web): advance session last-seen while selected session updates

Keep the unread watermark in sync with live session activity so
navigating away from an open session does not show a false unread dot.

Addresses review on tiann#699

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: heavygee

web/src/lib/sessionLastSeen.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it, beforeEach } from 'vitest'
+import { getSessionLastSeenAt, markSessionSeen } from './sessionLastSeen'
+
+describe('sessionLastSeen', () => {
+    beforeEach(() => {
+        localStorage.clear()
+    })
+
+    it('stores the latest seen timestamp for a session', () => {
+        markSessionSeen('session-a', 1000)
+        markSessionSeen('session-a', 2500)
+        expect(getSessionLastSeenAt('session-a')).toBe(2500)
+    })
+
+    it('does not move the watermark backwards', () => {
+        markSessionSeen('session-a', 5000)
+        markSessionSeen('session-a', 2000)
+        expect(getSessionLastSeenAt('session-a')).toBe(5000)
+    })
+})

web/src/router.tsx

@@ -150,11 +150,16 @@ function SessionsPage() {
     }, [machines])
     const sessionMatch = matchRoute({ to: '/sessions/$sessionId', fuzzy: true })
     const selectedSessionId = sessionMatch && sessionMatch.sessionId !== 'new' ? sessionMatch.sessionId : null
+    const selectedSession = useMemo(
+        () => sessions.find((session) => session.id === selectedSessionId) ?? null,
+        [sessions, selectedSessionId]
+    )
     useEffect(() => {
-        if (selectedSessionId) {
-            markSessionSeen(selectedSessionId)
+        if (!selectedSessionId) {
+            return
         }
-    }, [selectedSessionId])
+        markSessionSeen(selectedSessionId, Math.max(Date.now(), selectedSession?.updatedAt ?? 0))
+    }, [selectedSessionId, selectedSession?.updatedAt])
     const isSessionsIndex = pathname === '/sessions' || pathname === '/sessions/'
     const sidebar = useSidebarResize()
     const handleNewSessionInDirectory = useCallback((args: { machineId: string | null; directory: string }) => {