fix(profile): read and store the date of birth as a calendar day

The iOS app sends the date of birth as the instant its date picker
returned, and the profile writers stored that instant as sent. Since the
report cover and the share page read the date part of the stored value,
an entry made just after midnight in Berlin printed the day before, and a
Californian entry made in the evening printed the day after.

Both profile writers now store the calendar day the value names in the
owner's zone, at noon UTC like other date-only values, so an iOS client
reading the instant back in its own zone keeps the date. Older rows keep
their instant: dateOfBirthKey reads a date-only anchor (UTC midnight or
noon) by its date and any other instant in the owner's zone. The report
collector, the targets age, and the web forms that send the date back
read it through that helper.

src/components/onboarding/baseline-form.tsx

@@ -9,6 +9,8 @@ import {
   type BaselineFieldErrors,
 } from "@/components/onboarding/baseline-fields";
 import { useUnitDisplay } from "@/hooks/use-unit-display";
+import { dateOfBirthKey } from "@/lib/tz/date-only";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import {
   resolveHeightUnitAdapter,
   type HeightDraft,
@@ -59,6 +61,8 @@ interface BaselineInitialValues {
   heightCm: number | null;
   dateOfBirth: string | null;
   gender: string | null;
+  /** The owner's zone, which a date of birth from the iOS app is read in. */
+  timezone?: string;
 }
 
 interface BaselineFormState {
@@ -133,8 +137,12 @@ export function BaselineForm({
   const [form, setForm] = useState<BaselineFormState>(() => ({
     displayName: "",
     height: heightAdapter.toDraft(initial.heightCm),
-    // The payload's ISO instant, as the date field's YYYY-MM-DD.
-    dateOfBirth: initial.dateOfBirth ? initial.dateOfBirth.slice(0, 10) : "",
+    dateOfBirth: initial.dateOfBirth
+      ? dateOfBirthKey(
+          new Date(initial.dateOfBirth),
+          initial.timezone ?? DEFAULT_TIMEZONE,
+        )
+      : "",
     gender: initial.gender ?? "",
   }));
   const [saving, setSaving] = useState(false);

src/components/onboarding/confirm-screen.tsx

@@ -35,6 +35,8 @@ import {
   questionsToPassBeforeConfirm,
 } from "@/lib/onboarding/wizard-steps";
 
+const NO_BASELINE = { heightCm: null, dateOfBirth: null, gender: null };
+
 /**
  * v1.39 — the confirm screen.
  *
@@ -334,11 +336,7 @@ export function ConfirmScreen({
       ) : (
         <BaselineForm
           backHref={back ? screenHref(back) : undefined}
-          initial={{
-            heightCm: user?.heightCm ?? null,
-            dateOfBirth: user?.dateOfBirth ?? null,
-            gender: user?.gender ?? null,
-          }}
+          initial={user ?? NO_BASELINE}
           onConfirmed={finish}
           confirming={finishing}
           demoMode={demoMode}

src/components/settings/account-section/index.tsx

@@ -52,7 +52,7 @@ import {
 } from "./account-section-utils";
 import { AvatarSection } from "./avatar-section";
 import { SetupCard } from "./setup-card";
-import { dateOnlyKey } from "@/lib/tz/date-only";
+import { dateOfBirthKey } from "@/lib/tz/date-only";
 import { translateCodeOrNull } from "@/lib/i18n/translate-code";
 
 export { resolveInitialTimezone } from "./account-section-utils";
@@ -149,7 +149,7 @@ export function AccountSection() {
       displayName: user.displayName ?? "",
       height: heightAdapter.toDraft(user.heightCm),
       dateOfBirth: user.dateOfBirth
-        ? dateOnlyKey(new Date(user.dateOfBirth))
+        ? dateOfBirthKey(new Date(user.dateOfBirth), user.timezone)
         : "",
       gender: user.gender ?? "",
       fullName: user.fullName ?? "",