feat(systemd): add check_root service and timer

A new systemd service, `check_root`, has been added which checks disk usage of the root directory. If usage exceeds 85%, an email alert is sent. In addition to this service, a corresponding systemd timer is added to trigger this check daily.

systems/x86_64-linux/mx/default.nix

@@ -98,5 +98,36 @@
     };
   };
 
+  systemd.services = {
+    check_root = {
+      serviceConfig = {
+        Type = "oneshot";
+        Environment = "PATH=/run/current-system/sw/bin";
+        ExecStart = toString (
+          pkgs.writeShellScript "check_root.sh" ''
+            CURRENT=$(df / | grep / | awk '{ print $5}' | sed 's/%//g')
+            THRESHOLD=85
+
+            if [ "$CURRENT" -gt "$THRESHOLD" ] ; then
+                ${pkgs.mailutils}/bin/mail -s '/boot Disk Space Alert' harald << EOF
+            Your root partition remaining free space is critically low. Used: $CURRENT%
+            EOF
+            fi
+          ''
+        );
+      };
+      wantedBy = [ "default.target" ];
+    };
+  };
+
+  systemd.timers = {
+    check_root = {
+      timerConfig = {
+        OnCalendar = "daily";
+      };
+      wantedBy = [ "timers.target" ];
+    };
+  };
+
   system.stateVersion = "23.05";
 }