Migrate component previews from Widgetbook to widget previews

[knotbin]

Summary

Migrate the existing widgetbook workspace over to the repo's widget preview flow, then remove Widgetbook-specific dependencies and generated files.

Motivation

The app currently keeps component examples in a separate widgetbook workspace, with Widgetbook annotations, generator output, and a standalone WidgetbookApp. Moving these examples into widget previews should keep previews closer to the source widgets and reduce the extra workspace/codegen surface we maintain.

Context

Current Widgetbook surface includes:

• Root workspace entry: widgetbook
• Widgetbook package: widgetbook/pubspec.yaml
• App entrypoint: widgetbook/lib/main.dart
• App shell: widgetbook/lib/widgetbook.dart
• Generated directories: widgetbook/lib/widgetbook.directories.g.dart
• Preview/usecase files under widgetbook/lib/atoms, molecules, organisms, templates, tokens, and features/auth

Proposed direction

• Port the existing Widgetbook use cases to widget previews.
• Keep useful atoms/molecules/templates/auth onboarding coverage during the migration.
• Remove Widgetbook annotations, generator config, generated directories, and the widgetbook workspace once previews cover the same useful cases.
• Update setup/docs so codegen no longer references Widgetbook.

Acceptance criteria

• Existing useful Widgetbook scenarios are represented as widget previews.
• widgetbook is removed from the root workspace when no longer needed.
• Widgetbook dependencies and generated files are gone from the repo.
• Developer setup docs and common commands no longer mention Widgetbook codegen.