iOS: export/import stem playlists and sound settings

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

ios/Stemify/Playback/StemPlaylists.swift

@@ -70,6 +70,24 @@ final class StemPlaylists {
         return MixPresets.current(allStems, mix)?.label ?? "Custom Mix"
     }
 
+    /// Brings in playlists from an export: new ids are added, ids already here are replaced when the
+    /// import was updated more recently. Returns how many lists were added or replaced.
+    @discardableResult
+    func merge(_ incoming: [StemPlaylist]) -> Int {
+        var n = 0
+        for pl in incoming {
+            if let i = lists.firstIndex(where: { $0.id == pl.id }) {
+                guard pl.updated > lists[i].updated else { continue }
+                lists[i] = pl
+            } else {
+                lists.append(pl)
+            }
+            n += 1
+        }
+        if n > 0 { save() }
+        return n
+    }
+
     private func edit(_ id: String, _ change: (inout StemPlaylist) -> Void) {
         guard let i = lists.firstIndex(where: { $0.id == id }) else { return }
         change(&lists[i])