Keyring launchers, Firefox default browser, rootless Docker

- keyring.flatpaks / keyring.programs: desktop entries shadowing an app's
  own that run it on the outer bus (one kwalletd6 for both sessions), with
  the wallet's D-Bus names as `flatpak run --talk-name` options (no Flatpak
  overrides), --password-store=kwallet6 for Electron, and login callback
  schemes as default + recommended handlers.
- firefox.defaultBrowser: the launcher as default for http, https and
  text/html.
- docker: rootless dockerd as a user service, socket in the outer runtime
  dir, CLI with DOCKER_HOST for both sessions.

flake.nix

@@ -34,6 +34,8 @@
       };
       firefox = ./modules/firefox.nix;
       jellyfin = ./modules/jellyfin.nix;
+      keyring = ./modules/keyring.nix;
+      docker = ./modules/docker.nix;
     };
     systems = [ "aarch64-linux" "x86_64-linux" ];
     forSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});