feat: support helper and error text for select, checkbox, toggle, and radio group

[rdlabo]

Summary

• add helper and error text demos for checkbox, toggle, select, and radio group
• integrate select supporting text with inset-list border handling
• render radio-group supporting text below its items while preserving MD3 inset-list radii
• scope radio-group layout customization to structured inset lists

Validation

• npm run build
• cd demo && npm test -- --watch=false (34 tests)
• verified the radio helper/error layouts in the MD3 demo browser

[github-actions]

Playwright test results

 78 passed

Details

 78 tests across 1 suite
 1 minute, 33 seconds
 7864290
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-30/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.

[rdlabo]

/update-screenshots

[github-actions]

✅ Screenshots have been updated successfully!

The new screenshots have been committed to this PR.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-30/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.