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/components/NoteEditor.vue

@@ -7,6 +7,7 @@ import ColorPicker from "./ColorPicker.vue";
 import Icon from "./Icon.vue";
 import LabelPicker from "./LabelPicker.vue";
 import NoteChecklist from "./NoteChecklist.vue";
+import { fromLocalInput, toLocalInput } from "../notes/datetime";
 import type { Note, NoteLabel } from "../stores/notes";
 import type { NoteColor } from "../notes/colors";
 
@@ -86,6 +87,12 @@ async function openLink(link: { title: string; id: string | null }) {
   emit("navigate", created.id);
 }
 
+const reminderLocal = computed(() => toLocalInput(liveNote.value.remind_at));
+
+function onReminderChange(e: Event) {
+  void notes.setReminder(props.note.id, fromLocalInput((e.target as HTMLInputElement).value));
+}
+
 async function onLabelsChange(next: NoteLabel[]) {
   labelList.value = next;
   await notes.setLabels(
@@ -226,6 +233,24 @@ async function act(fn: () => Promise<void>) {
           </span>
         </div>
 
+        <div class="flex items-center gap-2 pt-1">
+          <Icon name="bell" class="text-neutral-400" />
+          <input
+            type="datetime-local"
+            :value="reminderLocal"
+            class="rounded-md border border-neutral-300 bg-white px-2 py-1 text-xs text-neutral-700 outline-none focus-visible:ring-2 focus-visible:ring-brand dark:border-neutral-700 dark:bg-neutral-800 dark:text-neutral-200"
+            @change="onReminderChange"
+          />
+          <button
+            v-if="liveNote.remind_at"
+            type="button"
+            class="text-xs text-neutral-400 hover:text-neutral-700 dark:hover:text-neutral-200"
+            @click="notes.setReminder(note.id, null)"
+          >
+            Clear
+          </button>
+        </div>
+
         <div
           v-if="outgoingLinks.length || backlinks.length"
           class="flex flex-col gap-2 border-t border-neutral-100 pt-2 dark:border-neutral-800"