feat(sessions): merge the activity feed into a tab on /sessions

Sessions and Activity Feed are two views of the same event pipeline, so
they now share one page: /sessions renders a tab bar (Sessions | Activity
Feed) with the active view in the ?view= query param, and /activity
redirects to /sessions?view=events. The sidebar entry is dropped.

The tab bar itself moves out of SettingsHub into components/PageTabs so
both hubs share it, and DocumentTitle loses the two branches for paths
that now only redirect.

client/README.md

@@ -444,9 +444,9 @@ useEffect(() => {
 graph TB
     Root["/"]
     Dashboard["/ (Dashboard)"]
-    Sessions["/sessions"]
+    Sessions["/sessions (tabs: sessions | events)"]
     Detail["/sessions/:id"]
-    Feed["/activity"]
+    Feed["/activity (redirect)"]
     Workflows["/workflows"]
     CcConfig["/cc-config (redirect)"]
     Run["/run"]
@@ -472,9 +472,10 @@ function App() {
       <Routes>
         <Route path="/" element={<Layout />}>
           <Route index element={<Dashboard />} />
-          <Route path="sessions" element={<Sessions />} />
+          <Route path="sessions" element={<History />} />
           <Route path="sessions/:id" element={<SessionDetail />} />
-          <Route path="activity" element={<ActivityFeed />} />
+          {/* legacy path - the activity feed is now a tab on /sessions */}
+          <Route path="activity" element={<Navigate to="/sessions?view=events" replace />} />
           <Route path="workflows" element={<Workflows />} />
           {/* legacy path - Claude config now lives in a Settings tab */}
           <Route path="cc-config" element={<Navigate to="/settings?tool=claude-code" replace />} />