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);
+});

src/app/utils/datetimeInput.ts

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+import { CSSProperties } from 'react';
+import { color as foldsColor, config as foldsConfig } from 'folds';
+
+const pad = (n: number): string => String(n).padStart(2, '0');
+
+// Format a Date as the value string expected by <input type="date"> in the
+// user's LOCAL timezone. (toISOString would shift to UTC and can land on the
+// wrong calendar day.)
+export function toLocalDate(date: Date): string {
+  return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`;
+}
+
+// Format a Date as the value string expected by <input type="time"> (local, HH:mm).
+export function toLocalTime(date: Date): string {
+  return `${pad(date.getHours())}:${pad(date.getMinutes())}`;
+}
+
+// Parse the local date + time <input> values back into a Date. Returns null when
+// either value is missing or the combination is invalid (e.g. a partial entry).
+export function parseLocalDateTime(dateValue: string, timeValue: string): Date | null {
+  if (!dateValue || !timeValue) return null;
+  const dt = new Date(`${dateValue}T${timeValue}:00`);
+  return Number.isNaN(dt.getTime()) ? null : dt;
+}
+
+// Shared style for date/time <input>s — matches the app's surface tokens and
+// hints a dark-mode calendar/clock popup via colorScheme.
+export function pickerInputStyle(
+  c: typeof foldsColor = foldsColor,
+  cfg: typeof foldsConfig = foldsConfig,
+): CSSProperties {
+  return {
+    background: c.SurfaceVariant.Container,
+    color: c.SurfaceVariant.OnContainer,
+    border: `${cfg.borderWidth.B300} solid ${c.SurfaceVariant.ContainerLine}`,
+    borderRadius: cfg.radii.R300,
+    padding: `${cfg.space.S200} ${cfg.space.S300}`,
+    fontSize: '0.875rem',
+    width: '100%',
+    boxSizing: 'border-box',
+    outline: 'none',
+    fontFamily: 'inherit',
+    colorScheme: 'dark',
+  };
+}