chore: switch workspace to pnpm

[markwylde]

Summary

• Replace npm workspaces with pnpm, including root scripts, workspace settings, lockfile, CI workflows, release automation, and Docker image build/install/prune steps.
• Update workspace dependencies and pnpm settings needed for reproducible pnpm 11 installs.
• Update docs, agent guidance, and user-facing command snippets from npm/npx to pnpm.

Verification

• pnpm install --frozen-lockfile
• npm run tidy
• npm run build
• docker build -t darkauth-pnpm-check .
• git diff --check

Notes

• Biome 2.5 still reports existing schema/deprecation infos and existing lint warnings during tidy.
• Vite still reports existing large chunk warnings during build.