Offer automatic package downloads, now that the machinery is installed

The page had a row for the case where dnf-automatic is absent and
nothing for the case where it is present, so installing it made the
setting disappear rather than appear.

The switch enables downloading only, which is the shipped default and
the right one to keep: a machine that installs packages unattended can
reboot into a kernel nobody chose. Downloading ahead of time is what
makes the install quick when someone does choose it, and the row says
exactly that rather than implying updates apply themselves.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/services/Updates.qml

@@ -106,6 +106,14 @@ Singleton {
         applyProcess.running = true;
     }
 
+    function setAutomaticDnf(enabled: bool): void {
+        if (applyProcess.running)
+            return;
+        root.lastError = "";
+        applyProcess.command = [root.helperPath, "set-auto-dnf", enabled ? "true" : "false"];
+        applyProcess.running = true;
+    }
+
     function setAutomaticFlatpak(enabled: bool): void {
         if (applyProcess.running)
             return;