iOS: iPhone Duo split layout at the hinge, 1.0 RC 3

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/UI/Nav.swift

@@ -51,6 +51,8 @@ final class Ui {
     var nowPlayingOpen = false {
         didSet { overlays.level = nowPlayingOpen ? 1 : 0 }
     }
+    /// True while an unfolded iPhone Duo shows Now Playing docked beside the library.
+    private(set) var split = false
     let overlays: Overlays
 
     init(overlays: Overlays) { self.overlays = overlays }
@@ -84,7 +86,14 @@ final class Ui {
         if !tabs.contains(tab) { tab = tabs.first ?? .Settings }
     }
 
-    func openNowPlaying() { nowPlayingOpen = true }
+    /// Docked Now Playing is already on screen, so there's nothing to open while split.
+    func openNowPlaying() { if !split { nowPlayingOpen = true } }
+
+    func setSplit(_ on: Bool) {
+        guard on != split else { return }
+        split = on
+        if on { nowPlayingOpen = false }
+    }
 
     /// Switches to the Library tab and pushes [r] there.
     func showInLibrary(_ r: Route) {