chore: update flake.lock (nixpkgs, Sovran_Bitcoin 1.1.1, Bisq2 2.1.14) and integration

- Update flake.lock inputs:
  - sovran-bitcoin: b0da63b -> 50a8b76 (1.1.1 with RTL 0.15.13 and Nix-native sovran-nwc 2.0.0)
  - nixpkgs (nixos-unstable): c59305b -> fe13242 (Caddy 2.11.7, Tor 0.4.9.14, GNOME Remote Desktop 50.2, Bitcoin Core 31.1, Electrs 0.12.0, LND 0.21.4-beta, BTCPay Server 2.4.5, Matrix Synapse 1.162.0, LiveKit 1.13.8, Vaultwarden 1.37.4, Haven 1.2.2)
  - nixpkgs-stable (nixos-26.05): 78e9c78 -> 26f4107
  - btc-clients: 99b0442 -> 990d29e (Bisq 1.10.9, Bisq2 2.1.14, Sparrow 2.5.5)
  - nixvim: 5980a62 -> 453dc9c
- flake.nix: replace deprecated pkgs.system with pkgs.stdenv.hostPlatform.system for btc-clients packages
- modules/sovran-bitcoin-integration.nix: guard config.networking.domain against null/empty when both haven and nwc-wallets features are enabled
- app/sovran_systemsos_web/versions.json: sync static fallback service versions with updated flake inputs
- CHANGELOG.md: document updates under [Unreleased]

flake.nix

@@ -51,9 +51,9 @@
 			];
 			config = {
 				environment.systemPackages = with pkgs; [
-					btc-clients.packages.${pkgs.system}.bisq
-					btc-clients.packages.${pkgs.system}.bisq2
-					btc-clients.packages.${pkgs.system}.sparrow
+					btc-clients.packages.${pkgs.stdenv.hostPlatform.system}.bisq
+					btc-clients.packages.${pkgs.stdenv.hostPlatform.system}.bisq2
+					btc-clients.packages.${pkgs.stdenv.hostPlatform.system}.sparrow
 				];
 			};
 		};