Function create_entry 

pub fn create_entry(
    state: &JournalState,
    fields: EntryFields,
) -> Result<PathBuf>

Create a new entry in journal with the given fields.

fields.title defaults to "" when None; fields.body defaults to "".

Fails with:

• Error::DuplicateTitle if another entry already has the same title.
• Error::DuplicateId if the generated ID already exists in the cache (extremely rare in practice).
• Error::EntryAlreadyExists if the destination file already exists on disk.
• Error::EntryNotFound / Error::EntryNotFoundByTitle if fields.parent cannot be resolved.