Stats: datetime localization improvements

[myhro]

Related to #76912.

Proposed Changes

• Drops the duplicated getMomentSiteZone() implementation;
• Adds locale information to theMoment instance;
• Tries first to use the site timezone;
• Defaults to the user browser timezone instead of UTC, if the site timezone isn't available.

The suggestion was to use withLocalizedMoment(), but that wouldn't work in the getMomentSiteZone() function. It returns a Moment instance and not an actual React component to be wrapped by a High-Order Component (HOC).

It was also suggested to use the current user timezone as an intermediate fallback, but this information doesn't actually exist (p1734669805865359-slack-C82FZ5T4G). We only have site and browser timezone information.

Why are these changes being made?

This PR implements the suggestions made on issue #76912, improving how datetime localization is handled.

Testing Instructions

I'm not aware of an example site that lacks the gmt_offset option, so the easiest way to simulate the fallback is to comment the line return localizedMoment.utcOffset( gmtOffset );. After that, it's possible to explore datetime information in the chart available at /stats/day/jetpack.com.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-fc8ee7a04faedbb0404c7034cf861828da5d2fff

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-fc8ee7a04faedbb0404c7034cf861828da5d2fff&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-fc8ee7a04faedbb0404c7034cf861828da5d2fff&env=a8c-for-agencies

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~71 bytes added 📈 [gzipped])

name                           parsed_size           gzip_size
plugins                              +57 B  (+0.0%)      +30 B  (+0.0%)
media                                +57 B  (+0.0%)      +30 B  (+0.0%)
jetpack-cloud-agency-sites-v2        +57 B  (+0.0%)      +37 B  (+0.0%)
backup                               +57 B  (+0.0%)      +37 B  (+0.0%)
activity                             +57 B  (+0.0%)      +37 B  (+0.0%)
a8c-for-agencies-sites               +57 B  (+0.0%)      +37 B  (+0.0%)
stats                                -24 B  (-0.0%)       +4 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~65 bytes added 📈 [gzipped])

name                                               parsed_size           gzip_size
async-load-store-app-store-stats-listview                +57 B  (+0.0%)      +35 B  (+0.0%)
async-load-store-app-store-stats                         +57 B  (+0.0%)      +35 B  (+0.0%)
async-load-design-playground                             +57 B  (+0.0%)      +30 B  (+0.0%)
async-load-design-blocks                                 +57 B  (+0.0%)      +30 B  (+0.0%)
async-load-design                                        +57 B  (+0.0%)      +30 B  (+0.0%)
async-load-calypso-post-editor-media-modal               +57 B  (+0.0%)      +30 B  (+0.0%)
async-load-calypso-post-editor-editor-media-modal        +57 B  (+0.0%)      +30 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[kangzj]

Nice refactoring! LGTM 👍

[myhro]

Thanks for taking the time to review, @kangzj. I'll merge/deploy it sometime after Christmas.