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/stores/notes.ts

@@ -34,6 +34,7 @@ export interface Note {
   pinned: boolean;
   archived: boolean;
   trashed: boolean;
+  remind_at: string | null;
   labels: NoteLabel[];
   items: ChecklistItem[];
   attachments: Attachment[];
@@ -95,7 +96,7 @@ export const useNotesStore = defineStore("notes", () => {
 
   async function mutate(
     id: string,
-    changes: Partial<Pick<Note, "title" | "body" | "color" | "kind" | "pinned" | "archived">>,
+    changes: Partial<Pick<Note, "title" | "body" | "color" | "kind" | "pinned" | "archived" | "remind_at">>,
   ): Promise<void> {
     reconcile(await api.patch<Note>(`/api/notes/${id}`, changes));
   }
@@ -104,6 +105,7 @@ export const useNotesStore = defineStore("notes", () => {
   const setArchived = (id: string, archived: boolean) => mutate(id, { archived });
   const setColor = (id: string, color: NoteColor) => mutate(id, { color });
   const setKind = (id: string, kind: NoteKind) => mutate(id, { kind });
+  const setReminder = (id: string, remindAt: string | null) => mutate(id, { remind_at: remindAt });
   const saveEdit = (id: string, changes: { title: string; body: string; color: NoteColor }) => mutate(id, changes);
 
   async function setLabels(id: string, labelIds: string[]): Promise<void> {
@@ -192,6 +194,7 @@ export const useNotesStore = defineStore("notes", () => {
     setArchived,
     setColor,
     setKind,
+    setReminder,
     saveEdit,
     setLabels,
     addItem,