Frontend: Split key/labels in ProfileTagInput

[tristanlabelle]

Refactors ProfileTagInput to take key:label pairs as the list of options, for example { en: "English", fr: "French" }. We still display the labels, but only expose the keys to consuming code since those are what the business logic needs. This way consumers don't need to map and reverse-map display strings.

This is related to and motivated by #8424: it will then make it trivial to translate spoken languages by providing a dictionary like { en: "Anglais", fr: "Français" } since we have translation keys for (some) language codes.

No functional changes.

Testing

Edited profile using the Vercel link on this PR.

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

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Backend

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[nabramow]

This seems okay to me, at least I tested and everything still works as before. I assume the actual ability to localize these will come in a follow up PR?

Do we also need to handle the dropdown where people choose the languages?:

[tristanlabelle]

@nabramow , yes, my next PR will bake localization in useRegions/useLanguages so it returns localized labels where available. The dropdown is already handled in this PR (See the <StyledAutocompleteOption> part).

[nabramow]

LGTM pulled the branch and nothing looks different at least

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR is explicitly a refactor with no functional changes and no visible user-facing impact yet. It prepares profile tag inputs for future localization work, but on its own it does not add a feature, fix an important user-facing bug, or introduce a major technical infrastructure improvement that end users would notice.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR is explicitly a refactor with no functional changes and no visible user-facing impact yet. It prepares profile tag inputs for future localization work, but on its own it does not add a feature, fix an important user-facing bug, or introduce a major technical infrastructure improvement that end users would notice.