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/DocumentTitle.tsx

@@ -24,12 +24,8 @@ export function DocumentTitle() {
     title = `${t("sessions")} · ${sessionId.slice(0, 8)}`;
   } else if (path.startsWith("/sessions")) {
     title = t("sessions");
-  } else if (path.startsWith("/activity")) {
-    title = t("activityFeed");
   } else if (path.startsWith("/workflows")) {
     title = t("workflows");
-  } else if (path.startsWith("/cc-config")) {
-    title = t("ccConfig");
   } else if (path.startsWith("/run")) {
     title = t("run");
   } else if (path.startsWith("/settings")) {