feat(dashboard): lazy routes, Needs-you strip, URL state, exact numbers

- Workspace, SettingsHub, SessionDetail and the Workflow sub-tab are lazy
  (main bundle 2,166 kB -> ~920 kB), wrapped in RouteErrorBoundary: a stale
  chunk after a self-update reloads once (vite:preloadError too), any other
  error shows a Reload panel instead of a blank page.
- Live refresh: lib/throttle.ts + hooks/useLiveRefresh.ts replace trailing
  debounces that froze under a steady event stream; refresh skips hidden
  tabs and refetches after a WebSocket reconnect. Monitor loads only while
  shown, with one batched GET /api/agents?session_ids= instead of N+1; a
  failing endpoint is an inline error, not a blank Dashboard.
- NeedsYouStrip on Monitor and above Analytics. Tabs, filters, page, sort
  and search live in the URL (Dashboard, Sessions, Activity, SessionDetail).
- Sessions: tool / workspace / date-range filters, CSV/JSON export
  (formula-neutralised), focusable row links, Open in workspace (the dead
  /run links are gone), stale responses dropped, error + Retry.
- Fixes: "NaNs" durations, 30/7-day charts skipping idle days, weekday
  labels west of UTC (now averages), cache hit rate denominator, billable
  tokens vs cache reads, exact tool-call counts, "To" date = end of day,
  invalid ?from= no longer throws, recent list no longer shrinks to 15,
  stable message keys, transcript switch races.
- Light-theme-unreadable bg-black code panels and dark-only DAG fills use
  tokens; status configs and chart colours use theme tokens; Health tab
  and other literals translated.
- Server (additive): GET /api/agents?session_ids=, GET /api/sessions
  awaiting=1&from=&to=, stats tool_call_count/distinct_tool_count, event
  rows carry session/agent labels.

client/src/pages/History.tsx

@@ -17,7 +17,7 @@ interface HistoryView {
   id: string;
   label: string;
   Icon: ComponentType<{ className?: string }>;
-  Panel: ComponentType;
+  Panel: ComponentType<{ embedded?: boolean }>;
 }
 
 /**
@@ -42,8 +42,10 @@ export function History() {
         activeId={active.id}
         onSelect={(id) => setParams(id === defaultView.id ? {} : { view: id }, { replace: true })}
         tabs={views.map(({ id, label, Icon }) => ({ id, label, Icon }))}
-      />
-      <active.Panel />
+      >
+        {/* `embedded`: the tab bar names the view, so the panel drops its own title. */}
+        <active.Panel embedded />
+      </PageTabs>
     </div>
   );
 }