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

hosts/chidi/default.nix

@@ -2,7 +2,7 @@
 	inputs,
 	pkgs,
 	user,
-	constants,
+	hostname,
 	...
 }: {
 	imports = [
@@ -18,8 +18,8 @@
 		inputs.sops-nix.darwinModules.sops
 	];
 
-	networking.hostName = "chidi";
-	networking.computerName = "Chidi";
+	networking.hostName = hostname;
+	networking.computerName = hostname;
 
 	home-manager.users.${user} = {
 		imports = [

hosts/chidi/secrets.nix

@@ -1,18 +1,14 @@
-{user, ...}: {
+{
+	user,
+	hostname,
+	...
+}: let
+	secrets = import ../../lib/secrets.nix;
+in {
 	sops.age.keyFile = "/Users/${user}/.config/sops/age/keys.txt";
 
-	sops.secrets = {
-		chidi-syncthing-cert = {
-			sopsFile = ../../secrets/chidi-syncthing-cert;
-			format = "binary";
-			owner = user;
-			path = "/Users/${user}/.config/syncthing/cert.pem";
-		};
-		chidi-syncthing-key = {
-			sopsFile = ../../secrets/chidi-syncthing-key;
-			format = "binary";
-			owner = user;
-			path = "/Users/${user}/.config/syncthing/key.pem";
-		};
+	sops.secrets = secrets.mkSyncthingSecrets {
+		inherit hostname user;
+		isDarwin = true;
 	};
 }