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.ts

@@ -23,6 +23,23 @@ export function parseLocalDateTime(dateValue: string, timeValue: string): Date |
   return Number.isNaN(dt.getTime()) ? null : dt;
 }
 
+// Human-friendly absolute time: "Today at 3:00 PM", "Tomorrow at 9:00 AM", or
+// "1/5/2026 at 3:00 PM". `now` is injectable so the relative-day logic is testable.
+export function formatFriendlyDateTime(ts: number, now: number = Date.now()): string {
+  const date = new Date(ts);
+  const nowDate = new Date(now);
+  const sameDay = (a: Date, b: Date): boolean =>
+    a.getFullYear() === b.getFullYear() &&
+    a.getMonth() === b.getMonth() &&
+    a.getDate() === b.getDate();
+  const tomorrow = new Date(nowDate);
+  tomorrow.setDate(tomorrow.getDate() + 1);
+  const timeStr = date.toLocaleTimeString(undefined, { hour: '2-digit', minute: '2-digit' });
+  if (sameDay(date, nowDate)) return `Today at ${timeStr}`;
+  if (sameDay(date, tomorrow)) return `Tomorrow at ${timeStr}`;
+  return `${date.toLocaleDateString()} at ${timeStr}`;
+}
+
 // 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(