Frontend/i18n: Default localizeDateTime to the browser's timezone.

[tristanlabelle]

Per @aapeliv's feedback. I was unconvinced by my own argument. :)

Left the explicit BROWSER_TIMEZONE in places where it's calling out #8064

Testing

Compared invite code dates on localhost vs next, they're the same.

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 18, 2026 8:41am

[aapeliv]

lgtm

[github-actions]

📝 Release Notes

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

Reason: This PR makes a small frontend date-formatting cleanup so most dates default to the browser's timezone without needing an explicit parameter. While it may slightly improve consistency of displayed dates, it is a minor implementation change rather than a clearly noticeable end-user feature, significant UX improvement, or major technical infrastructure change.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR makes a small frontend date-formatting cleanup so most dates default to the browser's timezone without needing an explicit parameter. While it may slightly improve consistency of displayed dates, it is a minor implementation change rather than a clearly noticeable end-user feature, significant UX improvement, or major technical infrastructure change.