feat: DSYS-476 Migrate AvatarIcon to ADR-0003 and ADR-0004

[cursor]

Description

Migrates the AvatarIcon component to follow ADR-0003 (String Unions) and ADR-0004 (Centralized Types Architecture).

• Creates AvatarIconSeverity const object (replacing the enum) and AvatarIconPropsShared type in @metamask/design-system-shared
• Exports the new shared types from the shared package index
• Updates both the React and React Native packages to import AvatarIconSeverity from the shared package instead of duplicating it in each platform's types/index.ts
• Removes the AvatarIconSeverity enum from both platform-specific types/index.ts files
• Updates all component files (constants, component implementation, stories, tests) to import from the correct source
• Updates Toast.types.ts in React Native which also referenced AvatarIconSeverity

Note: AvatarIconSize remains platform-specific as it is an alias for AvatarBaseSize, which has different values per platform (string labels in React, pixel numbers in React Native).

Related issues

Fixes: DSYS-476

Manual testing steps

1. Run yarn build from repo root — should complete with no errors
2. Run yarn test from repo root — all tests should pass with 100% coverage
3. Run yarn lint from repo root — no lint errors

Pre-merge author checklist

• I've followed MetaMask Contributor Docs
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

  

[github-actions]

📖 Storybook Preview

• View PR Storybook