Storybook: add stories for all UI primitive components

[Ejirowebfi]

Description

Storybook is configured but only has a preview setup. The UI/ folder contains Input, Button, ConfirmModal, ProgressIndicator, and InsufficientBalanceWarning with no stories.

Tasks

• Create *.stories.tsx files for each component in frontend/src/components/UI/
• Cover default, disabled, loading, and error states for each component
• Add the Storybook build to the storybook.yml CI workflow
• Verify Storybook builds without errors

Acceptance Criteria

• Each UI primitive has at least 2 stories (default + one variant)
• npm run storybook starts without errors
• CI Storybook build passes