feat(desktop/firefox): add synced profile support and move desktop config

modules/nixos/desktop/browser/default.nix

@@ -0,0 +1,25 @@
+{ lib, config, ... }:
+let
+  inherit (lib) mkEnableOption mkOption;
+  inherit (lib.types) listOf str;
+  inherit (lib.lists) allUnique;
+  cfg = config.desktop.browser;
+in
+{
+  options.desktop.browser = {
+    firefox = {
+      enable = mkEnableOption "";
+      syncedProfiles = mkOption {
+        type = listOf str;
+        default = [ ];
+      };
+    };
+  };
+
+  config.assertions = [
+    {
+      assertion = allUnique cfg.firefox.syncedProfiles;
+      message = "desktop.browser.firefox.syncedProfiles has duplicate elements.";
+    }
+  ];
+}