fix(site): stabilize date params to break infinite query loop on agents/analytics (#23414)

## Problem

`/agents/analytics` showed an infinite loading spinner. The browser
devtools revealed repeated requests to the chat cost summary endpoint
with `start_date` and `end_date` shifting by a few milliseconds on each
request.

`AgentAnalyticsPage` called `createDateRange(now)` on every render. When
`now` is not passed (production), `createDateRange` falls through to
`dayjs()`, which produces a new millisecond-precision timestamp each
time. Those timestamps became part of the React Query key via
`chatCostSummary()`, so every render created a new query identity, fired
a new fetch, state-updated, re-rendered, and the cycle repeated. The
page never left the loading branch because no query result was ever
observed for the `current` key before it changed.

The same pattern existed in `InsightsContent`, where
`timeRangeToDates()` called `dayjs()` on every render and fed the result
into `prInsights()`.

Storybook didn't catch this because stories pass a fixed `now` prop,
keeping the date range stable.

## Fix

Anchor the date window once using `useState`'s lazy initializer, then
derive `start_date`/`end_date` from the stable anchor during render — no
`useEffect`, no memoization for correctness, just stable input → stable
query key.

- **`AgentAnalyticsPage`**: `const [anchor] = useState<Dayjs>(() =>
dayjs())`, then `createDateRange(now ?? anchor)`. The `now` prop still
takes priority so Storybook snapshots remain deterministic.
- **`InsightsContent`**: Collapses `timeRange` and its anchor into a
single `TimeRangeSelection` state object. A fresh anchor is captured
only when the user changes the selected range (event handler), not on
render. Clicking the already-selected range is a no-op.

site/src/pages/AgentsPage/AgentAnalyticsPage.tsx

@@ -1,7 +1,7 @@
 import { chatCostSummary } from "api/queries/chats";
 import { useAuthContext } from "contexts/auth/AuthProvider";
 import dayjs, { type Dayjs } from "dayjs";
-import type { FC } from "react";
+import { type FC, useState } from "react";
 import { useQuery } from "react-query";
 import { AgentAnalyticsPageView } from "./AgentAnalyticsPageView";
 import { AgentPageHeader } from "./components/AgentPageHeader";
@@ -23,7 +23,8 @@ interface AgentAnalyticsPageProps {
 
 const AgentAnalyticsPage: FC<AgentAnalyticsPageProps> = ({ now }) => {
 	const { user } = useAuthContext();
-	const dateRange = createDateRange(now);
+	const [anchor] = useState<Dayjs>(() => dayjs());
+	const dateRange = createDateRange(now ?? anchor);
 
 	const summaryQuery = useQuery({
 		...chatCostSummary(user?.id ?? "me", {

site/src/pages/AgentsPage/components/InsightsContent.tsx

@@ -1,28 +1,42 @@
 import { prInsights } from "api/queries/chats";
 import { Spinner } from "components/Spinner/Spinner";
-import dayjs from "dayjs";
+import dayjs, { type Dayjs } from "dayjs";
 import { type FC, useState } from "react";
 import { useQuery } from "react-query";
 import { type PRInsightsTimeRange, PRInsightsView } from "./PRInsightsView";
 
-function timeRangeToDates(range: PRInsightsTimeRange) {
-	const end = dayjs();
+type TimeRangeSelection = {
+	timeRange: PRInsightsTimeRange;
+	anchor: Dayjs;
+};
+
+function timeRangeToDates(range: PRInsightsTimeRange, anchor: Dayjs) {
 	const days = Number.parseInt(range, 10);
-	const start = end.subtract(days, "day");
+	const start = anchor.subtract(days, "day");
 	return {
 		start_date: start.toISOString(),
-		end_date: end.toISOString(),
+		end_date: anchor.toISOString(),
 	};
 }
 
 export const InsightsContent: FC = () => {
-	const [timeRange, setTimeRange] = useState<PRInsightsTimeRange>("30d");
-	const dates = timeRangeToDates(timeRange);
+	const [selection, setSelection] = useState<TimeRangeSelection>(() => ({
+		timeRange: "30d",
+		anchor: dayjs(),
+	}));
+	const dates = timeRangeToDates(selection.timeRange, selection.anchor);
 
 	const { data, isLoading, error } = useQuery(prInsights(dates));
 
-	const handleTimeRangeChange = (range: PRInsightsTimeRange) =>
-		setTimeRange(range);
+	const handleTimeRangeChange = (timeRange: PRInsightsTimeRange) =>
+		setSelection((current) =>
+			current.timeRange === timeRange
+				? current
+				: {
+						timeRange,
+						anchor: dayjs(),
+					},
+		);
 
 	if (isLoading) {
 		return (
@@ -49,7 +63,7 @@ export const InsightsContent: FC = () => {
 	return (
 		<PRInsightsView
 			data={data}
-			timeRange={timeRange}
+			timeRange={selection.timeRange}
 			onTimeRangeChange={handleTimeRangeChange}
 		/>
 	);