feat(niri): add workspace-taskbar support

Adds a per-workspace app-icon taskbar (Workspace class) to the
niri/workspaces module. Rebased onto master: integrated with the
existing window-rewrite feature so the {windows} and {total} format
replacements continue to work alongside the new taskbar.

include/modules/niri/workspace.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <gtkmm/box.h>
+#include <gtkmm/button.h>
+#include <gtkmm/image.h>
+#include <gtkmm/label.h>
+#include <json/value.h>
+
+#include <string>
+
+namespace waybar::modules::niri {
+
+class Workspaces;
+
+class Workspace {
+ public:
+  Workspace(const Json::Value& workspace_data, Workspaces& manager);
+  ~Workspace() = default;
+
+  Workspace(const Workspace&) = delete;
+  Workspace& operator=(const Workspace&) = delete;
+
+  Gtk::Button& button() { return button_; }
+  uint64_t id() const { return id_; }
+
+  void update(const Json::Value& workspace_data, const std::vector<Json::Value>& all_windows,
+              const std::string& windows_str, std::size_t total);
+
+ private:
+  void rebuildTaskbar(const std::vector<Json::Value>& my_windows);
+
+  Glib::RefPtr<Gdk::Pixbuf> loadIcon(const std::string& app_id, int size);
+
+  Workspaces& manager_;
+  uint64_t id_;
+
+  // Layout:  button_
+  //            └─ box_  (horizontal)
+  //                 ├─ label_        workspace label / icon
+  //                 └─ taskbar_box_  app icon buttons (shown only when taskbar enabled)
+  Gtk::Button button_;
+  Gtk::Box box_;   
+  Gtk::Label label_;
+  Gtk::Box taskbar_box_; 
+};
+
+}  // namespace waybar::modules::niri