Frontend/i18n: Move monthFormatter to localizeDateTime infra.

[tristanlabelle]

Aligns formatting months with the design for formatting other dates, so they all have the same parameters. Will also support using the caching from #8122

localizeDateTime can be used directly, but I also added a localizeYearMonth helper.

Assumes changes from #8124 , so please also review :)

Testing

Ran frontend locally

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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Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	Mar 23, 2026 11:42am

[aapeliv]

i'll let Nicole look at this!

[nabramow]

Looks good to me! Switched around a bunch of languages in ReferenceListItem and the AboutMe and all looked okay.

[tristanlabelle]

@nabramow Thanks for the review and testing. I did a tiny change to merge with my other PR so I'll need reapproval: 0051570

[github-actions]

📝 Release Notes

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

Reason: This PR is mainly an internal frontend i18n/date-formatting refactor that consolidates month formatting into existing localization infrastructure. It does not introduce a clear new feature, major UX change, or important user-facing bug fix, and any visible impact appears minor and mostly about implementation consistency.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR is mainly an internal frontend i18n/date-formatting refactor that consolidates month formatting into existing localization infrastructure. It does not introduce a clear new feature, major UX change, or important user-facing bug fix, and any visible impact appears minor and mostly about implementation consistency.