fix(insights): show the Rest Mode onset as a date, not a raw timestamp

The health score card passed the episode onset, an ISO instant, straight
into its sentence, so it read "Rest Mode active since
2025-08-04T09:55:00.000Z". It now shows the day that instant falls on in
the profile timezone, in the user's date format. The test that pinned the
raw value now pins the formatted day.

src/components/insights/__tests__/health-score-card.test.tsx

@@ -20,7 +20,11 @@ import {
   type Translate,
 } from "../health-score-pillar-detail";
 
-let mockAuthUser: { unitPreference: string; glucoseUnit: string } = {
+let mockAuthUser: {
+  unitPreference: string;
+  glucoseUnit: string;
+  timezone?: string;
+} = {
   unitPreference: "metric",
   glucoseUnit: "mg/dL",
 };
@@ -624,14 +628,21 @@ describe("<HealthScoreCard> composite states", () => {
     const html = render(
       <HealthScoreCard
         report={scoredReport({
-          restMode: { active: true, since: "2026-07-20", episodeCount: 1 },
+          restMode: {
+            active: true,
+            since: "2026-07-19T23:30:00.000Z",
+            episodeCount: 1,
+          },
         })}
         tension={{ band: "yellow", positive: ["sleep"], negative: ["rhr"] }}
         returnToBand={{ metricType: "RESTING_HEART_RATE", daysInside: 6 }}
       />,
     );
     const rest = atRest(html);
-    expect(rest).toContain("Rest Mode active since 2026-07-20");
+    // The onset is an instant; it reads as the day it falls on in the
+    // profile timezone, in the user's date format, never as raw ISO.
+    expect(rest).toMatch(/Rest Mode active since \d{2}\/\d{2}\/2026\./);
+    expect(rest).not.toContain("T23:30:00");
     expect(rest).toContain('data-slot="health-score-tension"');
     expect(rest).toContain('data-slot="health-score-return-to-band"');
     // The contributor key and the metric type are localised, never raw.
@@ -640,6 +651,28 @@ describe("<HealthScoreCard> composite states", () => {
     expect(rest).not.toContain(">rhr<");
   });
 
+  it("reads the Rest Mode onset as the day in the profile timezone", () => {
+    mockAuthUser = {
+      unitPreference: "metric",
+      glucoseUnit: "mg/dL",
+      timezone: "Europe/Berlin",
+    };
+    const html = render(
+      <HealthScoreCard
+        report={scoredReport({
+          restMode: {
+            active: true,
+            since: "2026-07-19T23:30:00.000Z",
+            episodeCount: 1,
+          },
+        })}
+      />,
+    );
+    const rest = atRest(html);
+    expect(rest).toMatch(/Rest Mode active since (07\/20\/2026|20\/07\/2026)/);
+    expect(rest).not.toContain("2026-07-19T");
+  });
+
   it("shows the weekly gain as a chip beside the label", () => {
     const html = render(<HealthScoreCard report={scoredReport()} />);
     expect(html).toContain('data-slot="health-score-card-label"');

src/components/insights/health-score-card.tsx

@@ -24,6 +24,8 @@ import {
   type PillarDetailContext,
 } from "@/components/insights/health-score-pillar-detail";
 import { useAuth } from "@/hooks/use-auth";
+import { formatDate } from "@/lib/date-format";
+import { DEFAULT_TIMEZONE, userDayKey } from "@/lib/tz/format";
 import { useUnitDisplay } from "@/hooks/use-unit-display";
 import { resolveGlucoseUnit } from "@/lib/glucose";
 import type {
@@ -31,7 +33,7 @@ import type {
   ScoreBand,
   ScorePillarResult,
 } from "@/lib/analytics/score/types";
-import { useTranslations } from "@/lib/i18n/context";
+import { useDateFormatPreference, useTranslations } from "@/lib/i18n/context";
 import { SCORE_PILLAR_LABEL_KEYS } from "@/lib/score-config/labels";
 import { cn } from "@/lib/utils";
 
@@ -265,7 +267,8 @@ export function HealthScoreCard({
   onRetry,
   className,
 }: HealthScoreCardProps) {
-  const { t } = useTranslations();
+  const { t, locale } = useTranslations();
+  const dateFormat = useDateFormatPreference();
   const { user } = useAuth();
   const [open, setOpen] = useState(false);
   const regionId = useId();
@@ -528,7 +531,16 @@ export function HealthScoreCard({
             className="text-muted-foreground text-xs"
           >
             {t("insights.healthScore.restMode", {
-              since: report.restMode.since ?? t("insights.healthScore.none"),
+              since: report.restMode.since
+                ? formatDate(
+                    userDayKey(
+                      new Date(report.restMode.since),
+                      user?.timezone || DEFAULT_TIMEZONE,
+                    ),
+                    dateFormat,
+                    locale,
+                  )
+                : t("insights.healthScore.none"),
             })}
           </p>
         ) : null}