feat: Add systemd service to monitor boot disk space

This commit introduces a new systemd service that runs daily to check the disk usage of the /boot partition. If utilization exceeds a set threshold, it triggers a warning email. This will ensure prompt alerts on critically low boot disk space, helping in maintaining a stable system.

systems/x86_64-linux/mx/default.nix

@@ -67,5 +67,36 @@
     ];
   };
 
+  systemd.services = {
+    check_boot = {
+      serviceConfig = {
+        Type = "oneshot";
+        Environment = "PATH=/run/current-system/sw/bin";
+        ExecStart = toString (
+          pkgs.writeShellScript "check_boot.sh" ''
+            CURRENT=$(df /boot | grep /boot | 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 /boot partition remaining free space is critically low. Used: $CURRENT%
+            EOF
+            fi
+          ''
+        );
+      };
+      wantedBy = [ "default.target" ];
+    };
+  };
+
+  systemd.timers = {
+    check_boot = {
+      timerConfig = {
+        OnCalendar = "daily";
+      };
+      wantedBy = [ "timers.target" ];
+    };
+  };
+
   system.stateVersion = "23.05";
 }