fix: remove leaked NotificationCenter observer in PVRootViewNavigationController

Observer was added unconditionally in _initThemeListener() but the deinit
only removed it when iOS < 17.0, which is never true on our iOS 17+ target.
Observer now always removed on dealloc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

PVUI/Sources/PVSwiftUI/RootView/PVRootViewNavigationController.swift

@@ -221,10 +221,7 @@ public final class PVRootViewNavigationController: UINavigationController {
         }
     }
 
-    // Don't forget to remove the observer when it's no longer needed
     deinit {
-        if #unavailable(iOS 17.0, tvOS 17.0) {
-            NotificationCenter.default.removeObserver(self)
-        }
+        NotificationCenter.default.removeObserver(self)
     }
 }