feat: add activity notifications panel with session feed

- Bell icon shows dot indicator for recent activity
- Dropdown panel shows recent sessions with status icons
- Links to session detail and sessions list page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/NotificationsPanel.tsx

@@ -0,0 +1,105 @@
+import { useState, useEffect, useRef } from 'react'
+import { Link } from 'react-router-dom'
+import { Bell, CheckCircle, Clock } from 'lucide-react'
+import { sessionsApi } from '@/api/sessions'
+import type { Session } from '@/types/session'
+
+function timeAgo(dateStr: string): string {
+  const diff = Math.floor((Date.now() - new Date(dateStr).getTime()) / 1000)
+  if (diff < 60) return 'just now'
+  if (diff < 3600) return `${Math.floor(diff / 60)}m ago`
+  if (diff < 86400) return `${Math.floor(diff / 3600)}h ago`
+  return `${Math.floor(diff / 86400)}d ago`
+}
+
+export function NotificationsPanel() {
+  const [open, setOpen] = useState(false)
+  const [sessions, setSessions] = useState<Session[]>([])
+  const [hasNew, setHasNew] = useState(false)
+  const ref = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    sessionsApi.list({ size: 8 })
+      .then(data => {
+        setSessions(data)
+        // Mark as "new" if any session was updated in the last hour
+        const oneHourAgo = Date.now() - 3600000
+        setHasNew(data.some(s => new Date(s.started_at).getTime() > oneHourAgo))
+      })
+      .catch(() => {})
+  }, [])
+
+  useEffect(() => {
+    const handler = (e: MouseEvent) => {
+      if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false)
+    }
+    if (open) document.addEventListener('mousedown', handler)
+    return () => document.removeEventListener('mousedown', handler)
+  }, [open])
+
+  return (
+    <div className="relative" ref={ref}>
+      <button
+        onClick={() => { setOpen(!open); setHasNew(false) }}
+        className="relative rounded-lg p-2 text-muted-foreground hover:bg-card hover:text-foreground transition-colors"
+        title="Notifications"
+      >
+        <Bell size={18} />
+        {hasNew && (
+          <span className="absolute right-1.5 top-1.5 h-2 w-2 rounded-full bg-primary" />
+        )}
+      </button>
+
+      {open && (
+        <div className="absolute right-0 z-50 mt-2 w-80 rounded-xl border border-border bg-card shadow-xl animate-scale-in">
+          <div className="flex items-center justify-between border-b border-border px-4 py-3">
+            <h3 className="text-sm font-heading font-semibold text-foreground">Activity</h3>
+            <Link
+              to="/sessions"
+              onClick={() => setOpen(false)}
+              className="text-[0.6875rem] text-muted-foreground hover:text-foreground"
+            >
+              View All
+            </Link>
+          </div>
+
+          {sessions.length === 0 ? (
+            <div className="px-4 py-8 text-center text-sm text-muted-foreground">
+              No recent activity
+            </div>
+          ) : (
+            <div className="max-h-72 overflow-y-auto divide-y divide-border">
+              {sessions.map(session => (
+                <Link
+                  key={session.id}
+                  to={`/sessions/${session.id}`}
+                  onClick={() => setOpen(false)}
+                  className="flex items-start gap-3 px-4 py-3 hover:bg-accent/50 transition-colors"
+                >
+                  <div className="mt-0.5">
+                    {session.completed_at ? (
+                      <CheckCircle size={16} className="text-emerald-400" />
+                    ) : (
+                      <Clock size={16} className="text-amber-400" />
+                    )}
+                  </div>
+                  <div className="min-w-0 flex-1">
+                    <p className="text-sm text-foreground truncate">
+                      {session.tree_snapshot?.name || 'Session'}
+                    </p>
+                    <p className="text-[0.6875rem] text-muted-foreground">
+                      {session.completed_at
+                        ? `Completed ${timeAgo(session.completed_at)}`
+                        : `Started ${timeAgo(session.started_at)}`}
+                      {session.client_name && ` · ${session.client_name}`}
+                    </p>
+                  </div>
+                </Link>
+              ))}
+            </div>
+          )}
+        </div>
+      )}
+    </div>
+  )
+}

frontend/src/components/layout/TopBar.tsx

@@ -1,6 +1,6 @@
 import { useState, useRef, useEffect, useCallback } from 'react'
 import { Link, useNavigate } from 'react-router-dom'
-import { Search, Zap, Bell, LogOut, User, Shield, Settings } from 'lucide-react'
+import { Search, Zap, LogOut, User, Shield, Settings } from 'lucide-react'
 import { useAuthStore } from '@/store/authStore'
 import { usePermissions } from '@/hooks/usePermissions'
 import { useWorkspaceStore } from '@/store/workspaceStore'
@@ -8,6 +8,7 @@ import { getWorkspaceLabels } from '@/constants/workspaceLabels'
 import { BrandLogo } from '@/components/common/BrandLogo'
 import { CommandPalette } from './CommandPalette'
 import { QuickLaunch } from './QuickLaunch'
+import { NotificationsPanel } from './NotificationsPanel'
 import { cn } from '@/lib/utils'
 
 export function TopBar() {
@@ -103,9 +104,7 @@ export function TopBar() {
           >
             <Zap size={18} />
           </button>
-          <button className="relative rounded-lg p-2 text-muted-foreground hover:bg-card hover:text-foreground transition-colors" title="Notifications">
-            <Bell size={18} />
-          </button>
+          <NotificationsPanel />
 
           {/* User avatar & menu */}
           <div className="relative ml-2" ref={menuRef}>