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.

server/routes/agents.js

@@ -19,8 +19,26 @@ router.get("/", (req, res) => {
   const session_id = req.query.session_id;
   const sources = parseSources(req);
 
+  // `session_ids=a,b,c` = every agent of those sessions in ONE call (the
+  // Dashboard's active trees), capped so the IN list stays small.
+  const sessionIds =
+    typeof req.query.session_ids === "string"
+      ? [...new Set(req.query.session_ids.split(",").map((s) => s.trim()))]
+          .filter(Boolean)
+          .slice(0, 200)
+      : null;
+
   let rows;
-  if (session_id) {
+  if (sessionIds) {
+    rows = sessionIds.length
+      ? db
+          .prepare(
+            `SELECT * FROM agents WHERE session_id IN (${sessionIds.map(() => "?").join(",")})
+             ORDER BY started_at DESC LIMIT ?`
+          )
+          .all(...sessionIds, limit)
+      : [];
+  } else if (session_id) {
     // A session belongs to exactly one source, so no extra source filter needed.
     rows = stmts.listAgentsBySession.all(session_id);
   } else if (sources) {