add agent-browser + BW browser auth sync for netty

- Add chromium to netty system packages
- Add home/agent-browser.nix: configures agent-browser to use nix
  chromium headless, installs via npm on first activation (Linux only)
- Add scripts/sync-bw-browser-auth.sh: imports all BW login items
  into agent-browser encrypted auth vault via --password-stdin
- Add just sync-browser-auth target

hosts/netty/configuration.nix

@@ -108,6 +108,7 @@ in
   virtualisation.docker.enable = true;
 
   environment.systemPackages = packageSets.extras ++ [
+    pkgs.chromium
     pkgs.php
   ];