feat(reminders): view and cancel a message's existing reminders

There was no way to see or cancel a reminder once set (removeReminder was only
called by the fire-and-forget monitor), and addReminder didn't dedupe, so a
message could silently accumulate duplicate reminders. The Remind Me dialog now
lists the reminders already set on that message (soonest first) each with a
cancel button.

- New shared, tested formatFriendlyDateTime(ts, now?) in utils/datetimeInput.ts
  (Today/Tomorrow/date + time).
- Per-row cancel busy-guard; inline "Could not cancel" on failure.

Also applies two nits from the custom-time review: focus the date input when the
custom picker is revealed, and clear the error when editing date/time.

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

src/app/utils/datetimeInput.test.ts

@@ -1,6 +1,11 @@
 import assert from 'node:assert/strict';
 import { test } from 'node:test';
-import { toLocalDate, toLocalTime, parseLocalDateTime } from './datetimeInput';
+import {
+  toLocalDate,
+  toLocalTime,
+  parseLocalDateTime,
+  formatFriendlyDateTime,
+} 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.
@@ -43,3 +48,23 @@ test('parseLocalDateTime: valid input yields the local wall-clock time', () => {
   assert.equal(parsed.getHours(), 9);
   assert.equal(parsed.getMinutes(), 3);
 });
+
+test('formatFriendlyDateTime: uses Today/Tomorrow/date prefixes', () => {
+  const now = new Date(2026, 0, 5, 10, 0).getTime();
+  const laterToday = new Date(2026, 0, 5, 15, 30).getTime();
+  const tomorrow = new Date(2026, 0, 6, 9, 0).getTime();
+  const nextWeek = new Date(2026, 0, 12, 9, 0).getTime();
+
+  assert.ok(formatFriendlyDateTime(laterToday, now).startsWith('Today at '));
+  assert.ok(formatFriendlyDateTime(tomorrow, now).startsWith('Tomorrow at '));
+  const other = formatFriendlyDateTime(nextWeek, now);
+  assert.ok(!other.startsWith('Today'));
+  assert.ok(!other.startsWith('Tomorrow'));
+  assert.ok(other.includes(' at '));
+});
+
+test('formatFriendlyDateTime: Tomorrow rolls over month/year boundaries', () => {
+  const nye = new Date(2026, 11, 31, 23, 0).getTime();
+  const jan1 = new Date(2027, 0, 1, 9, 0).getTime();
+  assert.ok(formatFriendlyDateTime(jan1, nye).startsWith('Tomorrow at '));
+});