fix(text-area): retain block background in read-only states

[sb123sb123]

Summary

Read-only states currently override the textarea's block appearance with a transparent background, so the control looks like the outline variant. Keep the muted block background for both readonly and disabled with appearance-readonly, and add a regression test for each state.

Fixes #3071

Validation

• Focused Text Area Chrome Headless specs: 39 passed using Karma/Vite with only the Text Area spec loaded. The shared local dependency link lacks the repository's reporter packages, so the focused run used a minimal config.
• ESLint on the changed style and test files passed.