feat: bloc Toggle/Section repliable + infrastructure embeddings par fragments

- Nouveau bloc Toggle : sections dépliables dans l'éditeur (slash menu + drag handle)
  - Transformer un bloc existant en section repliable via le menu d'action
  - Boutons désactiver (unwrap) et supprimer dans l'en-tête
  - i18n FR/EN complet
- Infrastructure embeddings par fragments (inspiré AppFlowy)
  - Table NoteEmbeddingChunk + index HNSW
  - Chunking sémantique (~1000 chars, overlap 200, dedup par hash)
  - Indexation incrémentale au save (createNote + updateNote + clip)
  - Queue concurrence 4, retry backoff exponentiel

memento-note/locales/en.json

@@ -2407,6 +2407,13 @@
     "slashTableDesc": "Insert a simple grid",
     "slashDatabase": "Structured View",
     "slashDatabaseDesc": "Embed your notebook's structured data",
+    "slashToggle": "Toggle Section",
+    "slashToggleDesc": "Create a collapsible section",
+    "toggleOpened": "Expanded section — click to collapse",
+    "toggleClosed": "Collapsed section — click to expand",
+    "toggleDelete": "Delete section",
+    "toggleUnwrap": "Disable toggle section",
+    "blockActionInsertToggle": "Turn into toggle section",
     "slashDiagram": "Diagram",
     "slashDiagramDesc": "Generate a flow or mindmap",
     "slashSlides": "Presentation",