fix: user status change chart accommodates DST (#22191)

closes https://github.com/coder/internal/issues/464

# Summary

This PR resolves a flaky test that was sensitive to DST transitions in
various time zones. The root of the flake was:
* a bug; the query and its tests assume 24 hours per day
* the tests used local system time, which resulted in failures for dates
proximal to DST transitions

# Changes

Query:

The original query assumed 24 hour intervals between each day, which is
not a valid assumption. It now increments `1 day` at a time.

Database tests:

Database level tests for the query all assumed 24 hour days. They now
increment in DST-aware days instead. Instead of using time.Now() as a
base for testing, the test uses a series of dates over the course of an
entire year, to ensure that DST transition dates are present in every
test run.

# API Endpoint

The endpoint that delivers the user status chart now accepts an IANA
timezone name as a parameter and passes it, keeping the existing offset
as a fallback, to the database query.

API level tests were added to ensure the correct response form and error
behaviour. Correctness of content is tested at the database level.

site/src/api/api.ts

@@ -2533,11 +2533,14 @@ class ApiMethods {
 		return response.data;
 	};
 
+	// Intl.DateTimeFormat().resolvedOptions().timeZone returns an IANA timezone
+	// name (e.g. "America/New_York") per ECMA-402. Go's time.LoadLocation and
+	// PostgreSQL's timezone() both accept IANA names, so these are compatible.
 	getInsightsUserStatusCounts = async (
-		offset = Math.trunc(new Date().getTimezoneOffset() / 60),
+		timezone = Intl.DateTimeFormat().resolvedOptions().timeZone,
 	): Promise<TypesGen.GetUserStatusCountsResponse> => {
 		const searchParams = new URLSearchParams({
-			tz_offset: offset.toString(),
+			timezone,
 		});
 		const response = await this.axios.get(
 			`/api/v2/insights/user-status-counts?${searchParams}`,

site/src/api/typesGenerated.ts

@@ -2838,11 +2838,11 @@ export interface GetInboxNotificationResponse {
 
 // From codersdk/insights.go
 export interface GetUserStatusCountsRequest {
+	readonly timezone: string;
 	/**
-	 * Timezone offset in hours. Use 0 for UTC, and TimezoneOffsetHour(time.Local)
-	 * for the local timezone.
+	 * Deprecated: Use Timezone instead. Offset is ignored when Timezone is provided.
 	 */
-	readonly offset: number;
+	readonly offset?: number;
 }
 
 // From codersdk/insights.go