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.ts

@@ -0,0 +1,45 @@
+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',
+  };
+}