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/router/index.ts

@@ -17,6 +17,7 @@ const router = createRouter({
         { path: "label/:id", name: "label", component: () => import("../views/BoardView.vue") },
         { path: "search", name: "search", component: () => import("../views/SearchView.vue") },
         { path: "graph", name: "graph", component: () => import("../views/GraphView.vue") },
+        { path: "reminders", name: "reminders", component: () => import("../views/RemindersView.vue") },
       ],
     },
     {