feat(reminders): custom date/time option in Remind Me

The Remind Me dialog only offered four fixed presets, so you couldn't set a
reminder for an arbitrary time. Add a "Custom time…" option that reveals date +
time pickers (validated >= 1 minute in the future) and sets the reminder at that
absolute timestamp.

Also extract the local date/time <input> helpers (toLocalDate, toLocalTime,
parseLocalDateTime, pickerInputStyle) into a shared, unit-tested
utils/datetimeInput.ts and reuse them in ScheduleMessageModal (deduped from an
inline copy) — identical output, now covered by tests.

Documents the previously-undocumented Message Reminders feature in
LOTUS_FEATURES.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/datetimeInput.test.ts

@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import { test } from 'node:test';
+import { toLocalDate, toLocalTime, parseLocalDateTime } from './datetimeInput';
+
+// All helpers work in the LOCAL timezone. We construct Dates with the local
+// component constructor so the assertions hold regardless of where tests run.
+
+test('toLocalDate: zero-pads month and day', () => {
+  assert.equal(toLocalDate(new Date(2026, 0, 5, 9, 3)), '2026-01-05');
+  assert.equal(toLocalDate(new Date(2026, 11, 25, 0, 0)), '2026-12-25');
+});
+
+test('toLocalTime: zero-pads hours and minutes (24h)', () => {
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 9, 3)), '09:03');
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 23, 59)), '23:59');
+  assert.equal(toLocalTime(new Date(2026, 0, 5, 0, 0)), '00:00');
+});
+
+test('toLocalDate/toLocalTime round-trip through parseLocalDateTime', () => {
+  const d = new Date(2026, 5, 15, 14, 30, 0, 0);
+  const parsed = parseLocalDateTime(toLocalDate(d), toLocalTime(d));
+  assert.ok(parsed);
+  assert.equal(parsed.getTime(), d.getTime());
+});
+
+test('parseLocalDateTime: returns null for missing parts', () => {
+  assert.equal(parseLocalDateTime('', '09:00'), null);
+  assert.equal(parseLocalDateTime('2026-01-05', ''), null);
+  assert.equal(parseLocalDateTime('', ''), null);
+});
+
+test('parseLocalDateTime: returns null for an invalid combination', () => {
+  assert.equal(parseLocalDateTime('not-a-date', '09:00'), null);
+  assert.equal(parseLocalDateTime('2026-13-40', '09:00'), null);
+});
+
+test('parseLocalDateTime: valid input yields the local wall-clock time', () => {
+  const parsed = parseLocalDateTime('2026-01-05', '09:03');
+  assert.ok(parsed);
+  assert.equal(parsed.getFullYear(), 2026);
+  assert.equal(parsed.getMonth(), 0);
+  assert.equal(parsed.getDate(), 5);
+  assert.equal(parsed.getHours(), 9);
+  assert.equal(parsed.getMinutes(), 3);
+});