feat(profile): persist and expose the date-format preference

Accept dateFormat on the profile PATCH field-by-field, echo it from
the profile + /me reads, mirror it into the client auth user, and add
the field to the OpenAPI request/response schemas.

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

@@ -96,6 +96,10 @@ export const GET = apiHandler(async () => {
     // Hour-cycle display preference (AUTO follows the locale convention,
     // H12 / H24 pin the cycle). Clients mirror this into their formatters.
     timeFormat: user.timeFormat ?? "AUTO",
+    // Date-order display preference (AUTO follows the locale convention,
+    // DMY / MDY / YMD pin the field order). Clients mirror this into their
+    // formatters and the <DateField> primitive.
+    dateFormat: user.dateFormat ?? "AUTO",
     lastReportPracticeName: user.lastReportPracticeName ?? null,
     // v1.4.47 W3 — per-user Coach opt-out. Default `false` if the
     // column is absent (partial-deploy rollback safety, see migration

src/app/api/user/profile/route.ts

@@ -42,6 +42,7 @@ export const GET = apiHandler(async () => {
         locale: true,
         timezone: true,
         timeFormat: true,
+        dateFormat: true,
         moodReminderEnabled: true,
         fullName: true,
         insurerName: true,
@@ -77,6 +78,7 @@ export const GET = apiHandler(async () => {
     locale: dbUser?.locale ?? null,
     timezone: dbUser?.timezone ?? "Europe/Berlin",
     timeFormat: dbUser?.timeFormat ?? "AUTO",
+    dateFormat: dbUser?.dateFormat ?? "AUTO",
     moodReminderEnabled: dbUser?.moodReminderEnabled ?? false,
     fullName: dbUser?.fullName ?? null,
     insurerName: dbUser?.insurerName ?? null,
@@ -114,6 +116,7 @@ export const PATCH = apiHandler(async (request: NextRequest) => {
     locale: result.user.locale,
     timezone: result.user.timezone,
     timeFormat: result.user.timeFormat,
+    dateFormat: result.user.dateFormat,
     moodReminderEnabled: result.user.moodReminderEnabled,
     fullName: result.user.fullName,
     insurerName: result.user.insurerName,

src/components/settings/__tests__/disable-coach-card.test.tsx

@@ -68,6 +68,7 @@ function buildUser(disableCoach: boolean): AuthUser {
     glucoseUnit: null,
     unitPreference: "metric",
     timeFormat: "AUTO",
+    dateFormat: "AUTO",
     disableCoach,
     fullName: null,
     insurerName: null,

src/components/settings/__tests__/modules-section.test.tsx

@@ -70,6 +70,7 @@ function buildUser(modules: AuthUser["modules"]): AuthUser {
     glucoseUnit: null,
     unitPreference: "metric",
     timeFormat: "AUTO",
+    dateFormat: "AUTO",
     disableCoach: false,
     fullName: null,
     insurerName: null,

src/components/settings/__tests__/time-format-select.test.tsx

@@ -46,6 +46,7 @@ function buildUser(timeFormat: TimeFormatPreference): AuthUser {
     glucoseUnit: null,
     unitPreference: "metric",
     timeFormat,
+    dateFormat: "AUTO",
     disableCoach: false,
     fullName: null,
     insurerName: null,

src/components/settings/__tests__/unit-preference-select.test.tsx

@@ -49,6 +49,7 @@ function buildUser(unitPreference: "metric" | "imperial"): AuthUser {
     glucoseUnit: null,
     unitPreference,
     timeFormat: "AUTO",
+    dateFormat: "AUTO",
     disableCoach: false,
     fullName: null,
     insurerName: null,

src/hooks/use-auth.ts

@@ -7,8 +7,12 @@ import { queryKeys } from "@/lib/query-keys";
 import { apiGet, apiFetchRaw } from "@/lib/api/api-fetch";
 import { retryOnceOnTransientError } from "@/lib/queries/retry-transient";
 import { useTranslations } from "@/lib/i18n/context";
-import type { TimeFormatPreference } from "@/lib/format-locale";
+import type {
+  TimeFormatPreference,
+  DateFormatPreference,
+} from "@/lib/format-locale";
 import { isTimeFormatPreference, storeTimeFormat } from "@/lib/time-format";
+import { isDateFormatPreference, storeDateFormat } from "@/lib/date-format";
 import type { ModuleKey } from "@/lib/modules/registry";
 import type { TourProgress } from "@/lib/onboarding/tour-progress";
 import { clearOfflineCachesForSessionEnd } from "@/lib/pwa/query-persister";
@@ -75,6 +79,14 @@ export interface AuthUser {
    * "AUTO" against a stale /me payload.
    */
   timeFormat: TimeFormatPreference;
+  /**
+   * Date-order display preference. AUTO follows the locale convention
+   * (de → dd.MM.yyyy, en → MM/dd/yyyy); DMY / MDY / YMD pin the field order
+   * regardless of locale. Mirrored into localStorage by `fetchMe` so
+   * `useFormatters()` and the `<DateField>` primitive render the same order.
+   * Coerced to "AUTO" against a stale /me payload.
+   */
+  dateFormat: DateFormatPreference;
   /**
    * v1.4.47 W3 — per-user Coach opt-out. When `true`, every Coach
    * mount point (`<LayoutCoachFab>`, `<LayoutCoachMount>`, the
@@ -157,6 +169,15 @@ async function fetchMe(): Promise<AuthUser> {
     ? data.timeFormat
     : "AUTO";
   storeTimeFormat(timeFormat);
+  // Date-order preference: coerce against a stale /me payload, then mirror
+  // into localStorage so `useFormatters()` and the `<DateField>` primitive
+  // (which cannot reach the query cache) render the same field order.
+  const dateFormat: DateFormatPreference = isDateFormatPreference(
+    data.dateFormat,
+  )
+    ? data.dateFormat
+    : "AUTO";
+  storeDateFormat(dateFormat);
   return {
     ...(data as AuthUser),
     disableCoach: data.disableCoach ?? false,
@@ -167,6 +188,7 @@ async function fetchMe(): Promise<AuthUser> {
     // without the field) so the display defaults to metric.
     unitPreference: data.unitPreference === "imperial" ? "imperial" : "metric",
     timeFormat,
+    dateFormat,
     // v1.15.0 — coerce against a stale /me payload so the cycle nav entry
     // stays hidden by default when the field is absent.
     cycleTrackingEnabled: data.cycleTrackingEnabled === true,

src/lib/auth/profile-update.ts

@@ -27,6 +27,9 @@ const extendedProfileSchema = profileSchema.extend({
   // Hour-cycle display preference. AUTO follows the locale convention,
   // H12 forces AM/PM, H24 forces 24-hour.
   timeFormat: z.enum(["AUTO", "H12", "H24"]).optional(),
+  // Date-order display preference. AUTO follows the locale convention,
+  // DMY pins day-month-year, MDY month-day-year, YMD ISO yyyy-MM-dd.
+  dateFormat: z.enum(["AUTO", "DMY", "MDY", "YMD"]).optional(),
 });
 
 export interface ApplyProfileResult {
@@ -43,6 +46,7 @@ export interface ApplyProfileResult {
     timezone: string;
     locale: string | null;
     timeFormat: "AUTO" | "H12" | "H24";
+    dateFormat: "AUTO" | "DMY" | "MDY" | "YMD";
     moodReminderEnabled: boolean;
     // v1.7.0 — patient-identity fields. `insuranceNumber` is returned in
     // plaintext (decrypted on read by the route) so the client can render
@@ -110,6 +114,7 @@ export async function applyProfileUpdate(
   if (data.locale !== undefined) updates.locale = data.locale;
   if (data.timezone !== undefined) updates.timezone = data.timezone;
   if (data.timeFormat !== undefined) updates.timeFormat = data.timeFormat;
+  if (data.dateFormat !== undefined) updates.dateFormat = data.dateFormat;
   if (data.moodReminderEnabled !== undefined) {
     updates.moodReminderEnabled = data.moodReminderEnabled;
   }
@@ -206,6 +211,7 @@ export async function applyProfileUpdate(
       timezone: updatedUser.timezone,
       locale: updatedUser.locale,
       timeFormat: updatedUser.timeFormat ?? "AUTO",
+      dateFormat: updatedUser.dateFormat ?? "AUTO",
       moodReminderEnabled: updatedUser.moodReminderEnabled,
       fullName: updatedUser.fullName,
       insurerName: updatedUser.insurerName,

src/lib/feature-flags/__tests__/coach-user-disable.test.tsx

@@ -95,6 +95,7 @@ function buildUser(disableCoach: boolean): AuthUser {
     glucoseUnit: null,
     unitPreference: "metric",
     timeFormat: "AUTO",
+    dateFormat: "AUTO",
     disableCoach,
     fullName: null,
     insurerName: null,

src/lib/openapi/routes/profile.ts

@@ -185,6 +185,12 @@ const profileUpdateRequest = z
       .describe(
         "Hour-cycle display preference. AUTO follows the locale convention, H12 forces AM/PM, H24 forces 24-hour.",
       ),
+    dateFormat: z
+      .enum(["AUTO", "DMY", "MDY", "YMD"])
+      .optional()
+      .describe(
+        "Date-order display preference. AUTO follows the locale convention, DMY pins day-month-year (dd.MM.yyyy), MDY pins month-day-year (MM/dd/yyyy), YMD pins ISO yyyy-MM-dd.",
+      ),
     moodReminderEnabled: z.boolean().optional(),
     fullName: z.string().max(120).nullable().optional(),
     insurerName: z.string().max(120).nullable().optional(),
@@ -222,6 +228,11 @@ const profileResponse = z
       .describe(
         "Hour-cycle display preference. AUTO follows the locale convention, H12 forces AM/PM, H24 forces 24-hour.",
       ),
+    dateFormat: z
+      .enum(["AUTO", "DMY", "MDY", "YMD"])
+      .describe(
+        "Date-order display preference. AUTO follows the locale convention, DMY pins day-month-year (dd.MM.yyyy), MDY pins month-day-year (MM/dd/yyyy), YMD pins ISO yyyy-MM-dd.",
+      ),
     moodReminderEnabled: z.boolean(),
     fullName: z.string().nullable(),
     insurerName: z.string().nullable(),
@@ -261,6 +272,11 @@ const profileUpdateResponse = z
       .describe(
         "Hour-cycle display preference. AUTO follows the locale convention, H12 forces AM/PM, H24 forces 24-hour.",
       ),
+    dateFormat: z
+      .enum(["AUTO", "DMY", "MDY", "YMD"])
+      .describe(
+        "Date-order display preference. AUTO follows the locale convention, DMY pins day-month-year (dd.MM.yyyy), MDY pins month-day-year (MM/dd/yyyy), YMD pins ISO yyyy-MM-dd.",
+      ),
     moodReminderEnabled: z.boolean(),
     fullName: z.string().nullable(),
     insurerName: z.string().nullable(),