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/lib/format.ts

@@ -163,7 +163,7 @@ import i18n from "../i18n";
  *   parseDate("2026-04-18T08:49:13Z")    // parsed as-is
  *   parseDate("2026-04-18T08:49:13-04:00") // parsed as-is (explicit offset)
  */
-function parseDate(iso: string): Date {
+export function parseDate(iso: string): Date {
   // Already has timezone info (Z or +/- offset) - parse directly
   // (`/[+-]\d{2}:\d{2}$/` catches trailing `+04:00`-style offsets).
   if (/[Zz]$/.test(iso) || /[+-]\d{2}:\d{2}$/.test(iso)) {
@@ -297,6 +297,16 @@ export function formatDuration(start: string, end: string): string {
   return formatMs(ms);
 }
 
+/**
+ * A URL/user date value as an ISO string for an API param, or `undefined` when
+ * it is empty or unparseable (`new Date("x").toISOString()` throws).
+ */
+export function isoOrUndefined(value: string | null | undefined): string | undefined {
+  if (!value) return undefined;
+  const d = new Date(value);
+  return Number.isNaN(d.getTime()) ? undefined : d.toISOString();
+}
+
 /**
  * Formats a millisecond duration as the coarsest two-unit representation:
  * "Nh Nm" once >= 1 hour, "Nm Ns" once >= 1 minute, else "Ns".
@@ -311,6 +321,7 @@ export function formatDuration(start: string, end: string): string {
  *   formatMs(-10)       // "0s"
  */
 export function formatMs(ms: number): string {
+  if (Number.isNaN(ms)) return "-"; // unparseable input upstream
   if (ms < 0) return "0s"; // clamp negative spans (clock skew) to zero
   const totalSec = Math.floor(ms / 1000);
   const hours = Math.floor(totalSec / 3600); // whole hours
@@ -334,6 +345,7 @@ export function formatMs(ms: number): string {
  */
 export function timeAgo(iso: string): string {
   const ms = Date.now() - parseDate(iso).getTime();
+  if (Number.isNaN(ms)) return "-";
   const seconds = Math.floor(ms / 1000);
   if (seconds < 60) return i18n.t("common:time.justNow");
   const minutes = Math.floor(seconds / 60);