Saving in the tasks sheet ran `onCreate`/`onUpdate` in a `try/finally`
without a `catch`, so a refused create or update became an unhandled
rejection: the spinner stopped, nothing was shown, and the user could
not tell whether the task was saved.
The editor state and the save logic move to `src/lib/task-editor.ts`.
`submitTaskEditor` returns the server's reason instead of throwing; the
sheet shows it in an alert and leaves the editor untouched so the input
can be fixed or saved again. It is cleared only after a successful save.
