chore(pnpm): prepare pnpm 12 upgrade

[gcko]

PR checklist

• Ensure you have added or ran the appropriate tests for your PR.
• DCO signed

What type of PR is this?

chore

What this PR does / why we need it:

Prepares the repository for pnpm 12 while keeping the upgrade deterministic:

• pins pnpm 12.0.0, including its registry integrity hash, in js/package.json
• records pnpm 12's platform-specific executable packages and integrity hashes in the lockfile
• migrates every GitHub Actions job to the native pnpm/setup action, pinned to an exact commit
• makes each CI job read the pnpm version from js/package.json and fail if the installed version differs
• updates local bootstrap commands, documentation, and pre-commit handling for pnpm 12's multi-document lockfile

This directly addresses the earlier pnpm 11 bad-build concern: CI and local Corepack installs cannot float to a different pnpm build, and the currently published pnpm 12.0.0 package plus all eight platform executable artifacts were checked against their registry metadata and integrity values. This protects this exact release; a future pnpm version would still require an explicit reviewed update.

Which issue(s) this PR fixes:

None.

Special notes for your reviewer:

This PR is intentionally a draft. Please keep it unmerged until pnpm 12 has seen broader adoption.

Validation completed locally:

• CI=true pnpm install --frozen-lockfile
• pnpm lint:fix
• pnpm type:check
• pnpm test (4,287 passed, 5 skipped)
• pnpm run build
• make install-frontend-requires
• actionlint -shellcheck=
• repository commit and push hooks

Full actionlint continues to report pre-existing ShellCheck warnings in untouched workflow scripts; the workflow syntax check passes.

Does this PR introduce a user-facing change?:

NONE