Add virtualization support and adjust defaults

- Disable printing by setting `printEnable = false`.
- Add a new `virualization` module enabling libvirtd, virt‑manager,
  qemuGuest and spice‑vdagentd; enable it on the desktop host.
- Change default applications: enable GIMP, disable Pixieditor.
- Bump Helium AppImage to version 0.14.4.1 with updated hash.
- Lower notification sound volume from 0.5 to 0.1.
- Refactor Bash and Fish aliases: `upd` now updates flakes,
  `upg` performs OS switch, and new helper aliases are added.

modules/core/virualization.nix

@@ -0,0 +1,18 @@
+{
+  lib,
+  config,
+  ...
+}: {
+  options.virualization.enable = lib.mkEnableOption "Enable QEMU/KVM";
+
+  config = lib.mkIf config.virualization.enable {
+    virtualisation.libvirtd.enable = true;
+
+    programs.virt-manager.enable = true;
+
+    services = {
+      qemuGuest.enable = true;
+      spice-vdagentd.enable = true;
+    };
+  };
+}