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/__tests__/Sidebar.test.tsx

@@ -33,7 +33,10 @@ describe("Sidebar", () => {
     renderSidebar(true);
     expect(screen.getByText("Dashboard")).toBeInTheDocument();
     expect(screen.getByText("Sessions")).toBeInTheDocument();
-    expect(screen.getByText("Activity Feed")).toBeInTheDocument();
+    // Activity Feed and Claude Config are no longer top-level entries: they
+    // moved into tabs on /sessions and /settings respectively.
+    expect(screen.getByText("Settings")).toBeInTheDocument();
+    expect(screen.queryByText("Activity Feed")).toBeNull();
   });
 
   it('should show "Live" when WebSocket is connected', () => {
@@ -60,7 +63,8 @@ describe("Sidebar", () => {
     const hrefs = links.map((link) => link.getAttribute("href"));
     expect(hrefs).toContain("/");
     expect(hrefs).toContain("/sessions");
-    expect(hrefs).toContain("/activity");
+    expect(hrefs).toContain("/settings");
+    expect(hrefs).not.toContain("/activity");
   });
 
   it("should render both language options in expanded mode", () => {