Yattee v2 rewrite

Yattee/Extensions/View+NavigationSubtitle.swift

@@ -0,0 +1,36 @@
+//
+//  View+NavigationSubtitle.swift
+//  Yattee
+//
+//  Conditionally applies .navigationSubtitle on iOS 26+ and macOS 26+.
+//
+
+import SwiftUI
+
+struct NavigationSubtitleModifier: ViewModifier {
+    let subtitle: String?
+
+    func body(content: Content) -> some View {
+        #if os(iOS)
+        if #available(iOS 26, *), let subtitle {
+            content.navigationSubtitle(subtitle)
+        } else {
+            content
+        }
+        #elseif os(macOS)
+        if #available(macOS 26, *), let subtitle {
+            content.navigationSubtitle(subtitle)
+        } else {
+            content
+        }
+        #else
+        content
+        #endif
+    }
+}
+
+extension View {
+    func navigationSubtitleIfAvailable(_ subtitle: String?) -> some View {
+        modifier(NavigationSubtitleModifier(subtitle: subtitle))
+    }
+}