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/src/App.tsx

@@ -39,9 +39,8 @@
  * - `./components/Layout`
  * - `./components/SplashScreen`
  * - `./pages/Dashboard`
- * - `./pages/Sessions`
+ * - `./pages/History`
  * - `./pages/SessionDetail`
- * - `./pages/ActivityFeed`
  * - `./pages/Workflows`
  * - `./pages/SettingsHub`
  * - `./pages/Workspace`
@@ -63,9 +62,8 @@ import { Layout } from "./components/Layout";
 import { DocumentTitle } from "./components/DocumentTitle";
 import { SplashScreen } from "./components/SplashScreen";
 import { Dashboard } from "./pages/Dashboard";
-import { Sessions } from "./pages/Sessions";
+import { History } from "./pages/History";
 import { SessionDetail } from "./pages/SessionDetail";
-import { ActivityFeed } from "./pages/ActivityFeed";
 import { Workflows } from "./pages/Workflows";
 import { SettingsHub } from "./pages/SettingsHub";
 import { Workspace } from "./pages/Workspace";
@@ -95,9 +93,9 @@ export default function App() {
         <Routes>
           <Route element={<Layout wsConnected={connected} />}>
             <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 />} />
+            <Route path="activity" element={<Navigate to="/sessions?view=events" replace />} />
             <Route path="workflows" element={<Workflows />} />
             <Route path="lanes" element={<Navigate to="/run" replace />} />
             <Route