M2 checklists: note kind + items backend + editor/card UI

- Migration 0006: notes.kind ('text'|'list') + note_items (text, checked,
  position). Item API: add/update(toggle)/delete/reorder; PATCH note kind; note
  responses include kind + items[] (merged in one query alongside labels).
- notes store: kind/items on Note, setKind/addItem/updateItem/deleteItem.
- NoteChecklist component (toggle/add/edit/delete items); rendered read-only-ish
  on cards (checkboxes toggle) and editable in the editor.
- Editor: convert text<->checklist (body lines become items on convert to list).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/models/note.py

@@ -40,6 +40,8 @@ class Note(Base):
     title: Mapped[str | None] = mapped_column(Text(), nullable=True)
     body: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
     color: Mapped[str] = mapped_column(Text(), nullable=False, server_default="default")
+    # 'text' (freeform body) or 'list' (a checklist of note_items).
+    kind: Mapped[str] = mapped_column(Text(), nullable=False, server_default="text")
     pinned: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     archived: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     # Soft delete: non-null => in Trash. Restore sets it back to null.
@@ -55,6 +57,7 @@ class Note(Base):
             "title": self.title,
             "body": self.body,
             "color": self.color,
+            "kind": self.kind,
             "pinned": self.pinned,
             "archived": self.archived,
             "trashed": self.deleted_at is not None,