Frontend/perf: Cache Intl.DateTimeFormat objects.

[tristanlabelle]

Adds a cache for Intl.DateTimeFormat objects since I read around the internet that it's recommended and this benchmark confirms it's much faster on my machine.

Test case name	Result
new Intl.DateTimeFormat.format()	new Intl.DateTimeFormat.format() x 23,943 ops/sec ±3.33% (56 runs sampled)
cached Intl.DateTimeFormat.format()	cached Intl.DateTimeFormat.format() x 976,681 ops/sec ±1.87% (62 runs sampled)

Unbounded cache size growth isn't a concern. This will clear on navigation and I expect the most different formats a given page will have will be in the low single digits.

Testing

Verified two different date formats can still be shown.

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 8:12am

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📝 Release Notes

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

Reason: This is a small frontend performance optimization that caches date formatter objects internally. While it may slightly improve efficiency, it does not introduce a user-visible feature, significant UX change, important bug fix, or major infrastructure improvement that end users would notice in release notes.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This is a small frontend performance optimization that caches date formatter objects internally. While it may slightly improve efficiency, it does not introduce a user-visible feature, significant UX change, important bug fix, or major infrastructure improvement that end users would notice in release notes.