[Button] Migrate to new beta colour tokens

[pomfrida]

Part of #5119 (phase 1 — colour). First component migrated onto the redefined tokens from the eds-tokens beta, serving as the test case for the migration pattern.

Scope

• Switch Button (/next) onto the expanded static colours — expanded scale + step-based state names (🎛 Review & Refine Colour Token values #4742, 🧩 Re‑evaluate token naming conventions #4744), replacing dynamic colour tokens with static ones where the migration prescribes it
• Apply the new alpha-channel disabled states (🎨 Implement Alpha‑Channel approach for disabled states #4741)
• Consume tokens via the bundled next/css/variables.css entry (Add a bundled variables.css to the Tokens Studio release workflow #5201) — this doubles as validation input for ADR-0010 (Validate the bundled CSS entry in beta and accept ADR-0010 before the flip #5202)

Definition of Done

• Button uses the new beta colour tokens (no references to the old colour tokens)
• Disabled states use the alpha-channel tokens
• Visual regression checked against current Button
• Learnings captured to guide the remaining component migrations

[claude]

These issues appear closely related to this Button colour-token migration:

• 🧩 Component migration (colour, then spacing & typography) #5119 — 🧩 Component migration (colour, then spacing & typography) (open): the parent tracking issue; this Button migration is its phase-1 test case.
• Swap dynamic colour → static internally #5118 — Swap dynamic colour → static internally (closed, not_planned): the internal dynamic → static colour swap that directly precedes the "replace dynamic tokens with static ones" work here.
• 🎨 Refine the EDS Colour & Token System #4740 — 🎨 Refine the EDS Colour & Token System (open): the overarching epic that the expanded static colours and alpha-channel disabled states originate from.

This comment was generated by an LLM and may be inaccurate. Please verify that the linked issues are actually relevant.