tidy(ui): translation cleanup and CI checks

[psychedelicious]

Summary

• Port clean translations script to node. It checks for unused translation strings that exist in en.json but are not referenced in codebase. Unfortunately going the other way round (finding missing translations) is very hard.
• Add it to frontend CI checks.

Related Issues / Discussions

n/a

QA Instructions

n/a

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[psychedelicious]

I let the new CI check run twice on the PR, before and after committing the cleaned translations file.

• Before (failed): https://github.com/invoke-ai/InvokeAI/actions/runs/17464119988/job/49595511873
• After (passed): https://github.com/invoke-ai/InvokeAI/actions/runs/17464209468/job/49595792843

So looks like it is working correctly