feat: auto-save 2s + indicateur save + reminders inline actions (compléter/snooze)

- Auto-save debounce 2s dans note-editor-context
- lastSavedAt state + setIsDirty(false) dans handleSave
- Indicateur toolbar: ✓ sauvegardé / ● non enregistré avec timer relatif
- Reminders sidebar: bouton ✓ compléter + bouton +1h snooze (hover inline)
- i18n: clés reminders.markDone/snooze1h + notes.savedJustNow/unsaved (15 locales)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

memento-note/locales/ko.json

@@ -280,7 +280,9 @@
     "saveFailed": "저장 실패",
     "search": "검색",
     "unarchived": "보관 해제됨",
-    "uploading": "업로드 중..."
+    "uploading": "업로드 중...",
+    "savedJustNow": "Saved",
+    "unsaved": "Unsaved changes"
   },
   "pagination": {
     "previous": "←",
@@ -1011,7 +1013,8 @@
     "todayAt": "오늘 {time}",
     "tomorrowAt": "내일 {time}",
     "clearCompleted": "클리어 완료",
-    "viewAll": "모든 알림 보기"
+    "viewAll": "모든 알림 보기",
+    "snooze1h": "Snooze 1 hour"
   },
   "notebook": {
     "create": "노트북 만들기",