refactor(waybar): register workspace window icons

configs/nixos/registry/window-icon.nix

@@ -0,0 +1,25 @@
+{ lib, util, ... }:
+{
+  checks = [
+    {
+      assertion = util.registry.predicates.uniquePerHost (entry: entry.value.class);
+      message = "Window icons must use unique class patterns per host.";
+    }
+  ];
+
+  type = lib.types.submodule {
+    options = {
+      class = lib.mkOption {
+        description = "Matches the window class reported by the compositor.";
+        type = lib.types.str;
+        example = ".*[Ff]irefox.*";
+      };
+
+      icon = lib.mkOption {
+        description = "Sets the Nerd Font codepoint shown for matching windows.";
+        type = lib.types.str;
+        example = "f269";
+      };
+    };
+  };
+}