fix(webkit): render field-textarea's helper text through HelperText

[gabriel-lisboa-azion]

Summary

• FieldTextarea renders its helper through HelperText instead of a hand-rolled <small>, the same contract FieldText already uses: the field resolves only the helper's kind (disabled → invalid → required → helper), and HelperText owns the type, colour and lock glyph.
• The spec's helper token rows go away, since HelperText owns them.

How to test

1. npx vitest run src/components/inputs/field-textarea from packages/webkit → all pass, including the new helper kind cases.
2. Storybook → Components/Inputs/FieldTextarea → Disabled / Invalid / Required → the helper line matches FieldText's in each state.

Notes

• Helper test ids follow FieldText's: …__helper stays, and …__helper-text / …__helper-icon become …__helper__text / …__helper__icon. Consumer tests that target the old ids need the new ones.