feat: add timezone switcher

[erickzhao]

This PR adds a simple timezone switcher to the Sleuth toolbar UI. This allows you to switch between the System and User TZ in the log table view.

Note that the toggle is disabled if the User and System TZ are the same. I just disabled it in the UI in this case, but can hide it instead if it makes the toolbar a bit less cluttered (open to feedback).

Screen.Recording.2025-09-29.at.16.16.51.mov

The date-fns dependency also got two major version bumps here so we could integrate the new @date-fns/tz integration.

[erickzhao]

After some feedback from @nocturnll, here are a few tweaks:

• The Range Picker is now TZ-aware. It also is refactored to use date-fns instead of the default dayjs library to align with the rest of the codebase.
• The TZ label is now inside the switch instead of on the side.
• Fixed a bug where state files wouldn't clear on resetting the app (2703d90), leading to timezones to persist for mobile logs despite not having a log-context.json file.
• That said, the switcher is hidden if log-context.json doesn't exist now.

[nocturnll]

lgtm