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/EventDetail.tsx

@@ -277,7 +277,7 @@ function SummaryBlock({
     refs.length > 0 ? t("eventDetail.seeDetailsBelow", { fields: refs.join(" · ") }) : null;
   return (
     <div className="border border-border rounded overflow-hidden bg-surface-3/30">
-      <div className="px-3 py-1 border-b border-border bg-black/20">
+      <div className="px-3 py-1 border-b border-border bg-surface-1">
         <span className="text-fg-muted text-[10px] uppercase tracking-wide font-semibold">
           {t("eventDetail.summary")}
         </span>
@@ -394,8 +394,8 @@ function CodeView({ value }: { value: unknown }) {
   const text = typeof value === "string" ? value : safeStringify(value);
 
   return (
-    <div className="relative bg-black/70 border border-border rounded font-mono text-[11px] overflow-hidden">
-      <div className="flex items-center justify-between px-3 py-1.5 border-b border-border bg-black/40">
+    <div className="relative bg-surface-0 border border-border rounded font-mono text-[11px] overflow-hidden">
+      <div className="flex items-center justify-between px-3 py-1.5 border-b border-border bg-surface-1">
         <span className="text-fg-muted text-[10px] uppercase tracking-wide">
           {typeof value === "string" ? "text" : Array.isArray(value) ? "array" : "json"}
         </span>