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/components/StatCard.tsx

@@ -54,6 +54,7 @@
  *
  * ----------------------------------------------------------------------------- */
 
+import { Link } from "react-router-dom";
 import type { LucideIcon } from "lucide-react";
 import { Tip } from "./Tip";
 import { StatValueSkeleton } from "./Skeleton";
@@ -74,6 +75,8 @@ interface StatCardProps {
   raw?: string;
   /** When true, render skeletons in place of value/trend. */
   loading?: boolean;
+  /** Drill-in: the whole card links here (e.g. the list the number counts). */
+  to?: string;
 }
 
 /**
@@ -88,9 +91,10 @@ export function StatCard({
   accentColor = "text-accent",
   raw,
   loading = false,
+  to,
 }: StatCardProps) {
-  return (
-    <div className="card p-5">
+  const body = (
+    <>
       <div className="flex items-center justify-between gap-3 mb-3">
         <span className="text-xs font-medium text-fg-muted uppercase tracking-wider truncate">
           {label}
@@ -109,6 +113,16 @@ export function StatCard({
           <span className="text-xs text-fg-muted mb-1 flex-shrink-0">{trend}</span>
         )}
       </div>
-    </div>
+    </>
+  );
+  return to ? (
+    <Link
+      to={to}
+      className="card p-5 block hover:bg-surface-4/40 transition-colors focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+    >
+      {body}
+    </Link>
+  ) : (
+    <div className="card p-5">{body}</div>
   );
 }