🎨 Implement Alpha‑Channel approach for disabled states

[emirgens]

Description

Define and apply an alpha-channel based approach for disabled states across EDS components. Per the 23 Jun decision, the static colour set is being expanded, and alpha-channel colours come in as part of that expansion — disabled states being the primary use.

Scope

• Colour values are tracked as part of the static-colour expansion in 🎛 Review & Refine Colour Token values #4742.
• This issue: apply the alpha-channel disabled-state approach consistently across components, during the component-migration pass.
• Designers + developers aligned on consistent usage.

Definition of Done

• Alpha-channel disabled-state approach documented
• Disabled states updated across all relevant components
• Visual regression tested
• Designers and developers aligned on consistent usage