iOS: Duo unfolded layout with right-edge tab rail, stems+queue deck and swap sides

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

ios/Stemify/UI/Nav.swift

@@ -53,6 +53,8 @@ final class Ui {
     }
     /// True while an unfolded iPhone Duo shows Now Playing docked beside the library.
     private(set) var split = false
+    /// Split only: the side opposite Now Playing shows the stems and queue (true) or the selected tab.
+    var deck = true
     let overlays: Overlays
 
     init(overlays: Overlays) { self.overlays = overlays }
@@ -67,6 +69,7 @@ final class Ui {
 
     /// Tapping the selected tab again pops it to its root.
     func selectTab(_ t: Tab) {
+        if split && deck { deck = false; tab = t; return }
         if t == tab { paths[t] = [] } else { tab = t }
     }
 
@@ -89,6 +92,9 @@ final class Ui {
     /// Docked Now Playing is already on screen, so there's nothing to open while split.
     func openNowPlaying() { if !split { nowPlayingOpen = true } }
 
+    /// Split only: brings the stems and queue back beside Now Playing.
+    func showDeck() { deck = true }
+
     func setSplit(_ on: Bool) {
         guard on != split else { return }
         split = on
@@ -98,6 +104,7 @@ final class Ui {
     /// Switches to the Library tab and pushes [r] there.
     func showInLibrary(_ r: Route) {
         nowPlayingOpen = false
+        deck = false
         let t = home
         tab = t
         paths[t, default: []].append(r)
@@ -106,6 +113,7 @@ final class Ui {
     /// Switches to Settings and opens [r] (the Equalizer) there.
     func showInSettings(_ r: Route) {
         nowPlayingOpen = false
+        deck = false
         tab = .Settings
         if paths[.Settings]?.last != r { paths[.Settings, default: []].append(r) }
     }