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/lib/validations/__tests__/measurement-reminders.test.ts

@@ -118,3 +118,19 @@ describe("measurementReminderTypeEnum (V3 — completeness)", () => {
     expect(parsed.success).toBe(true);
   });
 });
+
+describe("createMeasurementReminderSchema — first-due date", () => {
+  const base = { label: "Blood test", intervalDays: 365 };
+  it.each([
+    ["2026-10-01", true],
+    ["2026-10-01T00:00:00.000Z", true],
+    ["2026-10-01T00:00:00-07:00", true],
+    ["2026-10-01T00:00:00", false],
+    ["01.10.2026", false],
+  ])("anchorDate %s is accepted: %s", (anchorDate, ok) => {
+    expect(
+      createMeasurementReminderSchema.safeParse({ ...base, anchorDate })
+        .success,
+    ).toBe(ok);
+  });
+});

src/lib/validations/measurement-reminders.ts

@@ -143,13 +143,22 @@ const cadenceRefinement = (
   }
 };
 
+/**
+ * The first-due date: a calendar date (`YYYY-MM-DD`, read as that day in the
+ * user's zone) or a date-time (the instant it names).
+ */
+const anchorDateField = z.union([
+  z.iso.date(),
+  z.iso.datetime({ offset: true }),
+]);
+
 export const createMeasurementReminderSchema = z
   .object({
     label: z.string().trim().min(1).max(120),
     measurementType: measurementReminderTypeEnum.nullish(),
     intervalDays: z.number().int().min(1).max(3650).nullish(),
     rrule: rruleField.nullish(),
-    anchorDate: z.iso.datetime({ offset: true }).nullish(),
+    anchorDate: anchorDateField.nullish(),
     notifyHour: z.number().int().min(0).max(23).default(9),
     location: z.string().trim().max(200).nullish(),
     enabled: z.boolean().default(true),
@@ -177,7 +186,7 @@ export const updateMeasurementReminderSchema = z
     measurementType: measurementReminderTypeEnum.nullish(),
     intervalDays: z.number().int().min(1).max(3650).nullish(),
     rrule: rruleField.nullish(),
-    anchorDate: z.iso.datetime({ offset: true }).nullish(),
+    anchorDate: anchorDateField.nullish(),
     notifyHour: z.number().int().min(0).max(23).optional(),
     location: z.string().trim().max(200).nullish(),
     enabled: z.boolean().optional(),