Refactor: split ImportWizard into pure ImportPipeline and UI wrapper

ImportPipeline holds all data and logic (raw JSON, records, proposals,
model_name, build_model). ImportWizard wraps it with UI-only state
(step, cursor, message). Rename WizardState → WizardStep throughout.
Headless import in dispatch.rs now constructs ImportPipeline directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/import/mod.rs

@@ -1,5 +1,5 @@
 pub mod wizard;
 pub mod analyzer;
 
-pub use wizard::{ImportWizard, WizardState};
+pub use wizard::{ImportWizard, ImportPipeline, WizardStep};
 pub use analyzer::{FieldKind, FieldProposal, analyze_records};