Harden systemd user unit for ActivityWatch

[jackdbd]

This addresses #3 and #4.

For a quick report on the security of all systemd user units available on a system, run:

systemd-analyze security --user

For a detailed report on the security of the systemd user unit activitywatch.service, run:

systemd-analyze security --user activitywatch.service

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[jackdbd]

Regarding this issue...

...I thought I could wait a few seconds before starting aw-qt, as it's done in the borgmatic systemd user unit.
I tried adding this to the systemd unit, but it didn't work.:

# Delay start so the system tray becomes available
ExecStartPre = "${pkgs.coreutils}/bin/sleep 30s";

For now I am ok with having to click that annoying dialog, since ActivityWatch is otherwise working fine.

Related:

• Could not detect systray on Ubuntu ActivityWatch/activitywatch#258
• https://askubuntu.com/questions/101828/no-system-tray-detected-on-this-system