chore: Bump yarn to 4.12.0

[rekmarks]

Bumps yarn to version 4.12.0 (latest) in the hope of resolving some Dependabot problems and also because we might as well.

---

Note

Updates the monorepo to Yarn 4.12.0 and aligns config/lockfile accordingly.

• Bumps root packageManager to yarn@4.12.0 in package.json
• Updates yarn.config.cjs constraint to expect packageManager: yarn@4.12.0
• Regenerates yarn.lock entries (checksum/resolution adjustments for typescript patch ranges)

^{Written by Cursor Bugbot for commit 427d75b. This will update automatically on new commits. Configure here.}