Update dev dependencies and document cleanup

[digital-pro]

Summary

• update dev dependencies (keeping Pinia v2 and @pinia/testing v0.x)
• remove unused dev deps (firebase-admin/firebase-tools) and drop vite polyfills plugin
• add documentation for the dev vulnerability cleanup and next steps

Test plan

• npm run build:dev

Next steps

• if runtime issues appear in roar/core-tasks flows, consider a targeted polyfill or upstream fix
• consider upstreaming core-tasks to move off jscat 3.x / Babel 6 chain
• run npm run lint and npm test to validate toolchain upgrades

[CCuskley]

This is a lot to review at once - could we consider breaking this into separate PRs that address updating dev dependencies, removing unused dependencies, and adding documentation in separate PRs? This will allow us to review/test incrementally. Note it's also not passing existing tests so not generating a dev link.

[digital-pro]

This was started back in October with the only "code" being changed as package.json and eliminating a couple unused imports.

But as it sat around it got farther out of sync with main and rebasing, etc. cluttered it up completely. Now it's not even close to being right.

So if someone wants to pick up the theme (basically dump packages we aren't using for development) I think that'd be useful. The PR also includes a script that identifies un-used packages and produces a report.

[CCuskley]

Ok sounds good - I do think we need to pick this up, but will convert to draft for now and we can use this as a starting point later.