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

tests/test_notes.py

@@ -96,3 +96,9 @@ async def test_graph_requires_auth(app):
     client = app.test_client()
     resp = await client.get("/api/graph")
     assert resp.status_code == 401
+
+
+async def test_reminders_requires_auth(app):
+    client = app.test_client()
+    resp = await client.get("/api/notes/reminders")
+    assert resp.status_code == 401