refactor: split services into reusable services/ modules

- services/{samba,avahi,audiobookshelf,containers,caddy,tailscale}.nix
- common.nix grows firewall base + timezone; hosts import what they need
- jupiter/vm/vps import service modules; drop the jupiter/services.nix monolith
- each module opens its own firewall ports; caddy/tailscale shared by hosts
- verified: jupiter/vps/vbox eval + jupiter builds, config equivalent

services/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;
+    };
+  };
+}