iPad: adaptive sidebar+detail shell and Mac-style diff (remote Phases 1-2)

Grow the universal NucleicRemote app into a width-adaptive shell so a
regular-width iPad renders the macOS sidebar+detail IA while the iPhone
keeps its TabView -- both over the same RemoteStore projection (one host
authority, N renderers).

Phase 1 (adaptive shell):
- AdaptiveRootView branches on horizontalSizeClass: CompactRootView (the
  existing iPhone TabView, moved verbatim) vs SplitRootView
  (NavigationSplitView) on regular width.
- SplitSidebar: Home/Projects/To-dos/Settings destinations + sessions
  grouped under their projects (attention-sorted), connection chip footer.
- SplitDetail selects a destination or a session; a selected session reuses
  SessionDetailView keyed .id(sessionID) so switching drives open/close.
- RemoteStore.closeOpen(_:) is now id-guarded so a split-view A->B switch
  (onAppear(B) before onDisappear(A)) can't tear down B's fresh subscription.
- IdentityStore.deviceID idiom-tags the prefix (ipad-/iphone-) for new
  installs so the host lists a paired iPad correctly.

Phase 2 (width tuning + diff):
- readableColumn() caps+centers Home and the transcript on wide layouts;
  a no-op at phone/portrait width.
- SessionDiffView switches on available width (GeometryReader): a Mac-style
  two-pane diff (file list + selected file's patch) on wide/landscape, the
  phone stack otherwise. UnifiedPatch splits the combined patch per file.
  Read-only, same wire, no protocol change.
- Demo diff fixture now carries both files' patches.

iPhone layout and behavior unchanged. Builds clean; verified in the iPad
simulator (demo mode).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

NucleicRemote/NucleicRemote/Models/IdentityStore.swift

@@ -1,6 +1,9 @@
 import Foundation
 import Security
 import NucleicProtocol
+#if canImport(UIKit)
+import UIKit
+#endif
 
 /// What the phone pins about its Mac at pairing (SYNC §4.2): the host's static key (for IK
 /// reconnect), a display name, and the transport + connection hint from the QR — LAN
@@ -42,11 +45,22 @@ enum IdentityStore {
     static func deviceID() -> String {
         let defaults = UserDefaults.standard
         if let existing = defaults.string(forKey: deviceIDKey) { return existing }
-        let id = "iphone-" + UUID().uuidString.prefix(8).lowercased()
+        let id = deviceIDPrefix + UUID().uuidString.prefix(8).lowercased()
         defaults.set(id, forKey: deviceIDKey)
         return id
     }
 
+    /// Idiom-tagged prefix so the host lists a paired device with the right kind/icon
+    /// (`ipad-…` vs `iphone-…`). Only stamps *freshly generated* ids — an existing install
+    /// keeps whatever id it already persisted, so upgrading a phone never changes its identity.
+    private static var deviceIDPrefix: String {
+        #if canImport(UIKit)
+        return UIDevice.current.userInterfaceIdiom == .pad ? "ipad-" : "iphone-"
+        #else
+        return "iphone-"
+        #endif
+    }
+
     static func loadPairedHost() -> PairedHost? {
         guard let data = UserDefaults.standard.data(forKey: pairedHostKey) else { return nil }
         return try? JSONDecoder().decode(PairedHost.self, from: data)