feat: migrate from yarn to pnpm and adopt Helsinki base images

Triggers release-please after #503 landed on main as a squash merge
without a conventional commit prefix.

This release captures the changes already on main from #503
(commit 8f02f2c):

- Migrate from yarn to pnpm (pnpm@11.1.2 via corepack/packageManager,
  yarn.lock -> pnpm-lock.yaml, resolutions -> overrides, CI on
  ci-pnpm-node.yml, supply-chain guardrail minimumReleaseAge: 10080)
- Adopt Helsinki shared base images
  (ubi10/nodejs-24-pnpm-builder-base + ubi10/nginx-126-spa-standard);
  drop custom nginx config and scripts/env.sh now provided by base
  images; internal port 8000 -> 8080
- Bump to Node 24 / ubi10 (.nvmrc, engines.node >= 24, @types/node ^24,
  CI node-version 24.x)
- Add Hauki-specific nginx include for large_client_header_buffers
  (long OIDC Authorization headers from Tunnistamo)

Refs: HAUKI-785
See: #503

Author: timwessman