fix(reminders): read a first-due date sent as a date as that day in the user's zone

A reminder's first-due date could only be sent as a date-time, and the
web form sent the browser's local midnight, so a browser set to a zone
other than the profile's could move the day. The API now also takes a
bare calendar date and stores it as that day's midnight in the user's
zone, and the web form sends the date itself. A date-time still means the
instant it names.

src/components/measurement-reminders/vorsorge-section.tsx

@@ -181,11 +181,13 @@ function toDateInputValue(iso: string | null): string {
   return new Date(d.getTime() - offset * 60 * 1000).toISOString().slice(0, 10);
 }
 
-/** `<input type="date">` value → an ISO instant at local midnight, or null. */
+/**
+ * `<input type="date">` value → the calendar date itself, or null. The server
+ * reads a bare date as that day in the profile's zone, so a browser set to
+ * another zone cannot move the first-due day.
+ */
 function fromDateInputValue(value: string): string | null {
-  if (!value) return null;
-  const d = new Date(`${value}T00:00:00`);
-  return Number.isNaN(d.getTime()) ? null : d.toISOString();
+  return /^\d{4}-\d{2}-\d{2}$/.test(value) ? value : null;
 }
 
 interface FormState {