refactor: reduce duplication and improve consistency across hosts

- Centralize home-manager _module.args in profiles/nixos.nix via sharedModules
- Add lib/secrets.nix with mkSyncthingSecrets helper for DRY secret definitions
- Move syncthing device IDs to lib/constants.nix
- Standardize hostname handling (Darwin hosts now use hostname arg)
- Add missing networking.hostName to tahani
- Fix redundant string interpolations

lib/secrets.nix

@@ -0,0 +1,25 @@
+{
+	mkSyncthingSecrets = {
+		hostname,
+		user,
+		isDarwin,
+	}: let
+		homeDir =
+			if isDarwin
+			then "/Users/${user}"
+			else "/home/${user}";
+	in {
+		"${hostname}-syncthing-cert" = {
+			sopsFile = ../secrets/${hostname}-syncthing-cert;
+			format = "binary";
+			owner = user;
+			path = "${homeDir}/.config/syncthing/cert.pem";
+		};
+		"${hostname}-syncthing-key" = {
+			sopsFile = ../secrets/${hostname}-syncthing-key;
+			format = "binary";
+			owner = user;
+			path = "${homeDir}/.config/syncthing/key.pem";
+		};
+	};
+}