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/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,