Allow user to set timezone

[Timshel]

Hey,

This adds the ability for a user to set a Timezone which will replace the default one from the browser.

Worked on it mainly because Firefox with resist fingerprinting (Cf https://github.com/element-hq/element-web/issues/25006) set the timezone to UTC, and I'm not aware of an easy way to whitelist only specific domains.

Edit: probably should have searched more before coding but there is supposed to be a whitelist in Firefox RFP but for now I have issues with it :sweat_smile:. And this might be useful in other cases.

Checklist

• [ ] Tests written for new code (and old code if feasible).
• [ ] New or updated public/exported symbols have accurate TSDoc documentation.
• [x] Linter and other CI checks pass.
• [x] Sign-off given on the changes (see CONTRIBUTING.md).