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 = [