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/__tests__/DateTimePicker.test.tsx

@@ -0,0 +1,28 @@
+/**
+ * @file DateTimePicker.test.tsx
+ * @description An end-bound picker ("To") picks the very end of a day, so an
+ * event at 23:59:30 is still inside the range.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect, vi } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+import { DateTimePicker } from "../DateTimePicker";
+
+describe("DateTimePicker", () => {
+  it("endOfDay picks 23:59:59.999 of the chosen day", () => {
+    const onChange = vi.fn();
+    render(<DateTimePicker value="" onChange={onChange} endOfDay aria-label="To" />);
+    fireEvent.click(screen.getByRole("button", { name: "To" }));
+    fireEvent.click(screen.getAllByRole("button", { pressed: false })[0]!);
+    expect(onChange.mock.calls[0]![0]).toMatch(/T23:59:59\.999$/);
+  });
+
+  it("a start bound picks midnight", () => {
+    const onChange = vi.fn();
+    render(<DateTimePicker value="" onChange={onChange} aria-label="From" />);
+    fireEvent.click(screen.getByRole("button", { name: "From" }));
+    fireEvent.click(screen.getAllByRole("button", { pressed: false })[0]!);
+    expect(onChange.mock.calls[0]![0]).toMatch(/T00:00$/);
+  });
+});