Reorganize services/ into category subfolders

Group service modules by category (media, network, vpn, identity,
dev, desktop) to make the growing services/ dir easier to navigate.
containers.nix stays at the top level since it's a shared backend,
not a single-category service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

services/network/avahi.nix

@@ -0,0 +1,13 @@
+{ ... }:
+
+# mDNS/DNS-SD (advertise the host + shares on the LAN).
+{
+  services.avahi = {
+    enable = true;
+    nssmdns4 = true;
+    publish = {
+      enable = true;
+      userServices = true;
+    };
+  };
+}