fix(charts): cut days in the account's zone when a mount passes none

Several chart mounts (the medications page, the coach chat, the
recovery section) pass no zone, and the chart then fell back to a fixed
Europe/Berlin, so those charts cut and labelled days in Berlin for
every user whatever their profile said.

The chart now falls back to the signed-in account's zone, and only
before that has loaded to the shared DEFAULT_TIMEZONE the server's own
resolver ends on. /api/auth/me reports the zone the server resolves:
the stored one, or the instance default when the stored one is
unusable, so the client cuts days where the server does.

Refs #1026

src/app/api/auth/me/route.ts

@@ -49,6 +49,10 @@ import { syncMfaEnrollCookie } from "@/lib/auth/mfa-enrollment";
 import { buildAvatarUrl } from "@/lib/avatar";
 import { decrypt } from "@/lib/crypto";
 import { prisma } from "@/lib/db";
+import {
+  isValidTimezone,
+  resolveServerDefaultTimezone,
+} from "@/lib/tz/resolver";
 import { isCycleEnabled } from "@/lib/cycle/gate";
 import {
   resolveModuleMap,
@@ -238,7 +242,12 @@ export const GET = apiHandler(async () => {
     heightCm: user.heightCm,
     dateOfBirth: user.dateOfBirth,
     gender: user.gender,
-    timezone: user.timezone,
+    // The zone the server cuts this account's days in: the stored zone, or
+    // the instance default when the stored one is unusable. Clients bucket
+    // by this value, so it must be the resolved one, not the raw column.
+    timezone: isValidTimezone(user.timezone)
+      ? user.timezone
+      : await resolveServerDefaultTimezone(),
     onboardingCompletedAt: user.onboardingCompletedAt,
     onboardingTourCompleted: user.onboardingTourCompleted,
     // v1.39 (C1) — the needs-based setup flow, published additively:

src/components/charts/__tests__/health-chart-timezone.test.tsx

@@ -0,0 +1,53 @@
+import { describe, it, expect, vi } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+/**
+ * A chart mount that passes no `userTimezone` must still cut days in the
+ * signed-in user's zone. Several mounts (the medications page, the coach
+ * chat, the recovery section) pass none, and the component used to fall
+ * back to a fixed Europe/Berlin, so a user in Asia/Kolkata saw those charts
+ * bucketed and labelled in Berlin days while every other chart used their
+ * own. The zone now comes from the account, which the server resolves.
+ */
+
+let lastKey: unknown[] | undefined;
+
+vi.mock("@tanstack/react-query", () => ({
+  keepPreviousData: (previous: unknown) => previous,
+  useQuery: ({ queryKey }: { queryKey: unknown[] }) => {
+    lastKey = queryKey;
+    return { data: [], isLoading: false };
+  },
+  useQueryClient: () => ({
+    cancelQueries: () => Promise.resolve(),
+    getQueryData: () => undefined,
+    setQueryData: () => undefined,
+    invalidateQueries: () => Promise.resolve(),
+  }),
+  useMutation: () => ({ mutate: () => undefined, isPending: false }),
+}));
+
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({
+    isAuthenticated: true,
+    user: { id: "u1", timezone: "Asia/Kolkata" },
+    isLoading: false,
+  }),
+}));
+
+describe("<HealthChart> timezone", () => {
+  it("uses the account's zone when the mount passes none", async () => {
+    const { I18nProvider } = await import("@/lib/i18n/context");
+    const { HealthChart } = await import("../health-chart");
+
+    renderToStaticMarkup(
+      <I18nProvider initialLocale="en">
+        <HealthChart types={["ACTIVITY_STEPS"]} title="Steps" unit="" />
+      </I18nProvider>,
+    );
+
+    expect(lastKey).toBeDefined();
+    expect(lastKey).toContain("Asia/Kolkata");
+    expect(lastKey).not.toContain("Europe/Berlin");
+  });
+});

src/components/charts/health-chart.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { chartSeriesParams } from "@/components/charts/chart-series-request";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { useAuth } from "@/hooks/use-auth";
 import { useRecordCapabilities } from "@/hooks/use-record-capabilities";
@@ -214,10 +215,11 @@ interface HealthChartProps {
   /**
    * v1.4.25 W7b — per-user display timezone. When passed (mount sites
    * thread `useAuth().user?.timezone`), x-axis tick labels and the
-   * per-day bucket keys both render in the user's zone instead of the
-   * legacy Europe/Berlin pin. Defaults to "Europe/Berlin" so older
-   * callers that haven't yet adopted the prop keep their previous
-   * behaviour bit-for-bit.
+   * per-day bucket keys both render in the user's zone. When a mount
+   * passes none, the chart uses the signed-in account's zone (which
+   * `/api/auth/me` reports already resolved against the server default),
+   * and only before that has loaded the shared `DEFAULT_TIMEZONE` the
+   * server itself falls back to.
    */
   userTimezone?: string;
   /**
@@ -614,7 +616,7 @@ export function HealthChart({
   chartKey,
   annotations,
   verticalMarkers,
-  userTimezone = "Europe/Berlin",
+  userTimezone: userTimezoneProp,
   valueScale = 1,
   valueOffset = 0,
   onVisibleStats,
@@ -625,6 +627,9 @@ export function HealthChart({
   showDataTable = false,
 }: HealthChartProps) {
   const { isAuthenticated, user } = useAuth();
+  // A mount without the prop used to pin Europe/Berlin, so those charts
+  // cut days in Berlin for every user whatever their profile said.
+  const userTimezone = userTimezoneProp || user?.timezone || DEFAULT_TIMEZONE;
   const { t, locale } = useTranslations();
   const fmt = useFormatters();
   // Issue #490 — x-axis tick + tooltip date labels. Every timestamp this