Implement full CRUD from ViewModel; UI only dispatches intents

[TBA5854]

Centralize Create/Read/Update/Delete logic inside ToDoViewModel. Expose a single observable list (LiveData/Flow). UI must call ViewModel intents only (e.g., onAdd(title, desc), onToggle(id), onEdit(id, fields), onDelete(id)), with no data mutations in Activity/Composables. Add or fix VM methods (addNewToDo, updateToDo, toggleCompletionStatus, deleteToDo) and ensure all UI paths are wired to these. Remove any duplicated Activity state and verify state survives configuration changes.