Add an opacity/alpha design token set and adopt it for overlays and disabled states

[1nonlypiece]

📌 Description

The design system has color, spacing, shadow, motion, border, typography, and
z-index tokens, but no opacity scale. Modal backdrops, disabled controls, and
skeletons hardcode alpha values. This issue adds an opacity.json token file and
exposes it through the loader.

🎯 Requirements and Context

• New design-system/tokens/opacity.json in DTCG format (disabled, backdrop,
  hover, muted levels).
• Register the file in getAllTokens in
  design-system/src/utils/token-loader.ts.
• Adopt the backdrop opacity token in Modal and the disabled token in Field.

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b feature/opacity-tokens

2. Implement changes

• Add design-system/tokens/opacity.json; update
  design-system/src/utils/token-loader.ts.
• Adopt tokens in src/components/Modal.tsx and src/components/Field.tsx.
• Tests: extend design-system/src/__tests__/token-loader.test.ts and
  design-system/src/__tests__/tokens.test.ts.
• Docs: extend design-system/documentation/token-catalog.md.

3. Test and commit

• Run the design-system Jest suite and npm test.
• Cover edge cases: token shape validation, loader merge with new file,
  components consuming token values.

Example commit message

feat: add opacity token set and adopt in Modal and Field

✅ Guidelines

• Minimum 95% test coverage on the new/changed lines.
• DTCG-conformant token shape.
• Timeframe: 96 hours.

🏷️ Labels

type-feature · area-frontend · MAYBE REWARDED · GRANTFOX OSS · OFFICIAL CAMPAIGN

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[silentgeckoaudit3801]

I'd like to take this issue if it is still available. I will wait for assignment/maintainer confirmation before opening a PR, since recent GrantFox-style issues in this campaign appear to prefer assigned work before review. Planned scope: add DTCG opacity tokens, register them in the token loader, adopt backdrop/disabled opacity in Modal and Field, and cover the loader/token shape changes with focused tests and documentation.