M3 reminders: notes.remind_at + Reminders view

- Migration 0010: notes.remind_at (nullable tz). PATCH accepts remind_at
  (ISO set / null clear); GET /api/notes/reminders (soonest first, non-trashed);
  serialize includes remind_at.
- Frontend: datetime util (local<->ISO, format, overdue); notes store setReminder;
  editor datetime-local picker + clear; card reminder chip (overdue = red);
  sidebar Reminders entry + /reminders view.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/notes/datetime.ts

@@ -0,0 +1,28 @@
+// Reminder datetime helpers. Reminders are stored as ISO UTC; the editor's
+// <input type="datetime-local"> works in local time.
+
+export function toLocalInput(iso: string | null): string {
+  if (!iso) return "";
+  const d = new Date(iso);
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}`;
+}
+
+export function fromLocalInput(local: string): string | null {
+  if (!local) return null;
+  return new Date(local).toISOString();
+}
+
+export function formatReminder(iso: string | null): string {
+  if (!iso) return "";
+  return new Date(iso).toLocaleString(undefined, {
+    month: "short",
+    day: "numeric",
+    hour: "numeric",
+    minute: "2-digit",
+  });
+}
+
+export function isOverdue(iso: string | null): boolean {
+  return !!iso && new Date(iso).getTime() < Date.now();
+}