Add Brazilian Portuguese translations

[GabrielLidenor]

Summary

closes #3999
This PR adds and normalizes the Brazilian Portuguese locale for the site.

• Adds Brazilian Portuguese to the language selector
• Uses the pt_br route consistently for the Brazilian variant
• Updates the browser locale redirect to map pt-BR to pt_br
• Localizes the home page and locale wording to Brazilian Portuguese

This keeps the Brazil-specific locale distinct from the Portugal locale while preserving a clean, working route and selector behavior.

Screenshots

Verification

• bundle exec rake build ✅
• bundle exec rake test ✅
• bundle exec rake check:markup ✅
• bundle exec rake check:links ✅

The site build completed successfully and generated the search index without errors.

Notes

Even though I am Brazilian, I used github copilot to help me with the file improvements for the Brazilian Portuguese language.

[GabrielLidenor]

@hsbt reviews are welcomed.

No sure who else should I tag.

[GabrielLidenor]

@hsbt if you think PR is too big, I can split it into multiple smaller ones.

[GabrielLidenor]

@st0012 Hi there, I just wanna know your opinion about this PR .

If it is too big, even though it is just translations, I can create smaller ones.

I also volunteer to be the maintainer of the Brazilian Portuguese language for future translations 🫡

[hsbt]

I'm not familiar with how pt and pt_br relate, but as far as I checked with Claude, the existing pt pages are already written in Brazilian Portuguese, and the pt_br files here are copies of them with only lang and the links changed.

I think merging your wording changes into pt would be better. Do you have an opinion on whether we should instead make pt proper European Portuguese and add Brazilian Portuguese as pt_br? That would also need someone to write and maintain the European Portuguese side.

[GabrielLidenor]

@hsbt the differences between the too are relatively small and barely noticeable.

I agree with you, I will maintain the current Portuguese translation instead of creating a new language set for just small differences.

Closing this PR to open one focused on fixing the current translations of Portuguese.