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

profiles/darwin.nix

@@ -53,7 +53,7 @@
 	};
 
 	users.users.${user} = {
-		name = "${user}";
+		name = user;
 		home = "/Users/${user}";
 		isHidden = false;
 		shell = pkgs.fish;