ci: migrate to pnpm/setup

[edoardocavazza]

Summary

• Replace pnpm/action-setup with its successor pnpm/setup, which also installs the Node.js runtime and can run pnpm install, so actions/setup-node and the separate "Install project dependencies" step are dropped everywhere (main.yml, pr.yml, pages.yml) since install always ran immediately after setup.
• pnpm install --frozen-lockfile is now expressed via the require-lockfile: true input, which makes pnpm/setup run install with --frozen-lockfile.
• node-version: latest is now expressed via runtime: node@latest.
• pnpm store caching (cache: pnpm on setup-node) is preserved via the cache: true input of pnpm/setup.
• Verified compatibility: the repo's packageManager is pinned to pnpm@12.2.1, and pnpm/setup@v2 supports pnpm v11+.

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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Latest commit: a87bcd7

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