iOS: Duo layout fixes, swipe-up option, fuller backup restore

- DuoRail sits at the bottom of the strip, clear of the back buttons
- Setting to turn off swipe up for Playing Next
- Tighter Now Playing on iPhone Duo (folded and docked), regular phones unchanged
- Bigger stacked stem faders in the Duo deck
- Backup v2 restores song mixes, custom presets and stem playlist covers

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

ios/Stemify/UI/Nav.swift

@@ -98,7 +98,11 @@ final class Ui {
     func setSplit(_ on: Bool) {
         guard on != split else { return }
         split = on
-        if on { nowPlayingOpen = false }
+        if on {
+            nowPlayingOpen = false
+            // Only an iPhone Duo ever splits, so from here on folded Now Playing uses its tighter layout.
+            UserDefaults.standard.set(true, forKey: "duoSeen")
+        }
     }
 
     /// Switches to the Library tab and pushes [r] there.