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/components/Sidebar.tsx

@@ -88,7 +88,6 @@ function isUpdatePayload(x: unknown): x is UpdateStatusPayload {
 const NAV_KEYS = [
   { to: "/", icon: LayoutDashboard, key: "nav:dashboard" },
   { to: "/sessions", icon: FolderOpen, key: "nav:sessions" },
-  { to: "/activity", icon: Activity, key: "nav:activityFeed" },
   { to: "/workflows", icon: Workflow, key: "nav:workflows" },
   { to: "/run", icon: Play, key: "nav:workspace" },
   { to: "/settings", icon: Settings, key: "nav:settings" },