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

frontend/src/components/Icon.vue

@@ -14,6 +14,7 @@ const paths: Record<string, string> = {
   pencil: '<path d="M21.174 6.812a1 1 0 0 0-3.986-3.987L3.842 16.174a2 2 0 0 0-.5.83l-1.321 4.352a.5.5 0 0 0 .623.622l4.353-1.32a2 2 0 0 0 .83-.497z"/><path d="m15 5 4 4"/>',
   plus: '<path d="M5 12h14"/><path d="M12 5v14"/>',
   check: '<path d="M20 6 9 17l-5-5"/>',
+  checkbox: '<rect width="18" height="18" x="3" y="3" rx="2"/><path d="m9 12 2 2 4-4"/>',
 };
 </script>