fix(ui): return error from handleFormSubmit when no handler matches

Previously handleFormSubmit silently returned nil when handlerForFormKind
returned no handler, swallowing a potential programming error. Now it
returns a descriptive error so mismatches surface immediately.

closes #252

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

Author: cpcloud

internal/app/forms.go

@@ -1202,7 +1202,7 @@ func (m *Model) handleFormSubmit() error {
 	}
 	handler := m.handlerForFormKind(m.formKind)
 	if handler == nil {
-		return nil
+		return fmt.Errorf("no handler for form kind %v", m.formKind)
 	}
 	return handler.SubmitForm(m)
 }