fix(insights): plot the trends row and log fevers in the reader's units

Two more surfaces carried a fixed unit. The insights trends row labelled
weight in kg and body temperature in °C for every reader, and plotted
walking distance as raw metres under a km label; its trend caption
said the same. A single transformed series there now takes unit, scale
and offset from the preference, and the caption describes the converted
series against a converted noise floor.

The illness day sheet and timeline showed and took a fever in °C. The
fever is now seeded, typed and shown in the reader's temperature unit
and stored in °C as before.

Refs #1067

src/components/illness/__tests__/illness-day-timeline-units.test.tsx

@@ -0,0 +1,70 @@
+/**
+ * A logged fever reads in the reader's temperature unit. The timeline and the
+ * day sheet printed a hardcoded °C, so an imperial account read its own
+ * fever in Celsius.
+ */
+import { describe, expect, it, vi } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+const account = vi.hoisted(() => ({
+  user: { unitPreference: "imperial", glucoseUnit: null } as Record<
+    string,
+    unknown
+  >,
+}));
+
+vi.mock("@/hooks/use-auth", () => ({
+  useAuth: () => ({ user: account.user, isAuthenticated: true }),
+  useAccountOnceMounted: () => account.user,
+}));
+
+vi.mock("@/hooks/use-record-capabilities", () => ({
+  useRecordCapabilities: () => ({ canManageDomain: () => true }),
+}));
+
+vi.mock("../use-illness", () => ({
+  useIllnessDayLogList: () => ({
+    isLoading: false,
+    isError: false,
+    data: {
+      dayLogs: [
+        {
+          id: "d1",
+          episodeId: "e1",
+          date: "2026-09-30",
+          functionalImpact: null,
+          feverC: 38.5,
+          symptoms: [],
+          note: null,
+          updatedAt: "2026-09-30T08:00:00.000Z",
+        },
+      ],
+    },
+  }),
+}));
+
+import { I18nProvider } from "@/lib/i18n/context";
+import { IllnessDayTimeline } from "../illness-day-timeline";
+
+function render(): string {
+  return renderToStaticMarkup(
+    <I18nProvider initialLocale="en">
+      <IllnessDayTimeline episodeId="e1" onLogDay={() => {}} />
+    </I18nProvider>,
+  );
+}
+
+describe("<IllnessDayTimeline> fever unit", () => {
+  it("shows the fever in °F for an imperial reader", () => {
+    account.user = { unitPreference: "imperial", glucoseUnit: null };
+    const html = render();
+    // 38.5 °C = 101.3 °F.
+    expect(html).toContain("Temperature: 101.3 °F");
+    expect(html).not.toContain("°C");
+  });
+
+  it("keeps °C for a metric reader", () => {
+    account.user = { unitPreference: "metric", glucoseUnit: null };
+    expect(render()).toContain("Temperature: 38.5 °C");
+  });
+});

src/components/illness/illness-day-timeline.tsx

@@ -19,6 +19,7 @@ import { Badge } from "@/components/ui/badge";
 import { Button } from "@/components/ui/button";
 import { Skeleton } from "@/components/ui/skeleton";
 import { useTranslations, useFormatters } from "@/lib/i18n/context";
+import { useUnitDisplay } from "@/hooks/use-unit-display";
 
 import { ILLNESS_SYMPTOM_CATALOG } from "./symptom-catalog";
 import { useIllnessDayLogList } from "./use-illness";
@@ -32,6 +33,7 @@ function symptomLabelKey(key: string): string | null {
 function DayLogRow({ log }: { log: IllnessDayLogDTO }) {
   const { t } = useTranslations();
   const fmt = useFormatters();
+  const { toDisplay, unitFor } = useUnitDisplay();
 
   return (
     <div className="border-border/60 space-y-2 border-l-2 pl-3">
@@ -72,7 +74,10 @@ function DayLogRow({ log }: { log: IllnessDayLogDTO }) {
 
       {log.feverC !== null ? (
         <p className="text-foreground text-sm">
-          {t("illness.timeline.fever", { value: fmt.number(log.feverC, 1) })}
+          {t("illness.timeline.fever", {
+            value: fmt.number(toDisplay("BODY_TEMPERATURE", log.feverC), 1),
+            unit: unitFor("BODY_TEMPERATURE"),
+          })}
         </p>
       ) : null}
 

src/components/illness/log-day-sheet.tsx

@@ -25,6 +25,7 @@ import { Label } from "@/components/ui/label";
 import { Textarea } from "@/components/ui/textarea";
 import { cn } from "@/lib/utils";
 import { useTranslations } from "@/lib/i18n/context";
+import { useUnitDisplay } from "@/hooks/use-unit-display";
 
 import { ILLNESS_SYMPTOM_CATALOG } from "./symptom-catalog";
 import { useIllnessDayLog, useUpsertDayLog } from "./use-illness";
@@ -140,6 +141,10 @@ export function LogDaySheet({
   onsetDate,
 }: LogDaySheetProps) {
   const { t } = useTranslations();
+  // The fever is stored in °C and typed in the reader's temperature unit:
+  // seeded through the transform, inverted on save.
+  const { toDisplay, fromDisplay, unitFor } = useUnitDisplay();
+  const feverUnit = unitFor("BODY_TEMPERATURE");
 
   // The day being logged — seeded from `date` (today), but the picker lets the
   // user backdate within [onset, today] for the retrospective journal.
@@ -178,7 +183,11 @@ export function LogDaySheet({
       new Map((dto?.symptoms ?? []).map((s) => [s.key, s.severity ?? null])),
     );
     setFunctionalImpact(dto?.functionalImpact ?? null);
-    setFever(dto?.feverC != null ? String(dto.feverC) : "");
+    setFever(
+      dto?.feverC != null
+        ? String(toDisplay("BODY_TEMPERATURE", dto.feverC))
+        : "",
+    );
     setNote(dto?.note ?? "");
   } else if (!open && lastFormKey !== null) {
     setLastFormKey(null);
@@ -213,7 +222,10 @@ export function LogDaySheet({
     const input: IllnessDayLogInput = {
       date: selectedDate,
       functionalImpact,
-      feverC: feverValue,
+      feverC:
+        feverValue == null
+          ? null
+          : Math.round(fromDisplay("BODY_TEMPERATURE", feverValue) * 100) / 100,
       symptoms: Array.from(symptoms.entries()).map(([key, severity]) => ({
         key,
         severity: severity ?? undefined,
@@ -333,7 +345,7 @@ export function LogDaySheet({
           summary={
             feverValue != null ? (
               <span className="text-muted-foreground text-xs">
-                {feverValue.toFixed(1)} °C
+                {feverValue.toFixed(1)} {feverUnit}
               </span>
             ) : null
           }
@@ -345,7 +357,7 @@ export function LogDaySheet({
             <Input
               id="illness-fever"
               inputMode="decimal"
-              placeholder="37.0"
+              placeholder={toDisplay("BODY_TEMPERATURE", 37).toFixed(1)}
               value={fever}
               onChange={(e) => setFever(e.target.value)}
               className="max-w-32"