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/constants.nix

@@ -11,4 +11,10 @@
 		nixos = "25.11";
 		homeManager = "25.11";
 	};
+
+	syncthingDeviceIds = {
+		tahani = "6B7OZZF-TEAMUGO-FBOELXP-Z4OY7EU-5ZHLB5T-V6Z3UDB-Q2DYR43-QBYW6QM";
+		jason = "42II2VO-QYPJG26-ZS3MB2I-AOPVZ67-JJNSE76-U54CO5Y-634A5OG-ECU4YQA";
+		chidi = "N7W6SUT-QO6J4BE-T3Y65SM-OFGYGNV-TGYBJPX-JVN4Z72-AENZ247-KWXOQA6";
+	};
 }

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";
+		};
+	};
+}