fix(desktop): gate set_badge_label behind macOS cfg

Windows/Linux WebviewWindow have no set_badge_label method —
build failed on GitHub Actions windows runner.

apps/desktop/src-tauri/src/lib.rs

@@ -171,8 +171,10 @@ pub fn run() {
                     format!("ChatApp · {} neu", payload.count)
                 };
                 let _ = tray_handle.set_tooltip(Some(tooltip));
-                // macOS dock badge. `set_badge_label` is macOS-only but the
-                // call is a no-op on other platforms so we don't need a cfg.
+                // Dock/taskbar badge. `set_badge_label` is macOS-only.
+                // Windows taskbar uses an overlay icon (different API).
+                // Linux has no cross-DE badge API — skip.
+                #[cfg(target_os = "macos")]
                 if let Some(win) = badge_window.as_ref() {
                     let badge = if payload.count == 0 {
                         None
@@ -181,6 +183,8 @@ pub fn run() {
                     };
                     let _ = win.set_badge_label(badge);
                 }
+                #[cfg(not(target_os = "macos"))]
+                let _ = &badge_window;
             });
 
             Ok(())