if logged in, update user activity

[redshiftzero]

This PR updates the Cyd server with usage data in compliance with Lockdown Systems' Privacy Policy. An API request gets sent after the Plausible analytics data to Cyd server:

When you run this PR against https://github.com/lockdown-systems/cyd-server/pull/128 you'll see:

api-1         | 02-17 20:45 werkzeug     INFO     172.19.0.1 - - [17/Feb/2025 20:45:30] "POST /user/activity HTTP/1.1" 200 -

[micahflee]

This works great! Each time you open the Cyd app it updates the last accessed date.

However, if you open the Cyd app and you're not logged in, it should also update the last accessed date as soon as logging in is successful, somewhere around here: https://github.com/lockdown-systems/cyd/blob/main/src/renderer/src/modals/SignInModal.vue#L134-L154

Also, if we ever do #332, it's possible that someone might open Cyd and keep it open and actively in use for weeks but without updating the last accessed timestamp. Maybe there should be a setInterval that just automatically updates it (if logged in) every 6 hours or something. That could live in App.vue. So if the app is open in the background for days and days, we'll have the right day.