Add time format preference with system, 12, and 24 options

[larouxn]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here) - check:types fails due to ToastManager.dom.tsx, potentially related to 5bde700, but this is also failing on main so unrelated to this PR.
• My changes are ready to be shipped to users - Cannot say yes until I've seen the preference in a local build.

Description

This pull request aims to add a Time format preference with system, 12, and 24 options so that users can either leave it up to their locale to specify the time format (system) or manually select either 12 or 24 themselves. The preference should appears directly below Language and above Theme.

As for the implementation, the one thing that is currently bothering me a bit is the existence of both HourCyclePreference and HourCycleSetting. Unfortunately as the former is already in use for the original 24 hour macOS support implementation, shown below, we cannot settle on just one without refactoring the existing code. 🤔

Signal-Desktop/ts/types/I18N.std.ts

Lines 37 to 41 in 90c0fc7

	export enum HourCyclePreference {
	Prefer24 = 'Prefer24', // either h23 or h24
	Prefer12 = 'Prefer12', // either h11 or h12
	UnknownPreference = 'UnknownPreference',
	}

Sort of a follow up to 1143c0e, which added 24 hour time format for macOS.

Resolves #4079 (exactly)
Resolves #4252 (sort of, not exactly)

Testing

I added test coverage for this new preference. Additionally I tried running the app locally on my Fedora Workstation 43 (GNOME 49) system but unfortunately I'm seeing some GTK error with the local build. Not sure if that's due to an issue on my end or not.

Screencast.From.2025-11-24.18-14-59.mp4