ci: fix pnpm/setup config (invalid require-lockfile input)

[edoardocavazza]

Summary

The previous pnpm/setup migration (#53, already merged) used a require-lockfile input that does not actually exist on pnpm/setup@v2 (valid inputs are version, dest, runtime, cache, cache-dependency-path, package-json-file, install, token), which was silently ignored with a CI warning and left installs unpinned to the lockfile. This PR:

• Replaces require-lockfile: true with install: false on the pnpm/setup step plus an explicit pnpm install --frozen-lockfile step, restoring the original frozen-lockfile behavior.
• Re-adds runtime: node@latest on every pnpm/setup step.

Test plan

• Confirm CI (lint/build/test jobs, release, pages) passes on this branch

🤖 Generated with Claude Code

https://claude.ai/code/session_01NM1AyJH42derNim3BmJpVB

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