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)

NucleicRemote/NucleicRemote/Models/RemoteStore.swift

@@ -290,6 +290,17 @@ final class RemoteStore: ObservableObject {
             +  if (!token) throw new AuthError("missing bearer token")
                return verify(token)
              }
+            diff --git a/auth/session.ts b/auth/session.ts
+            new file mode 100644
+            --- /dev/null
+            +++ b/auth/session.ts
+            @@ -0,0 +1,6 @@
+            +export interface Session {
+            +  userId: string
+            +  issuedAt: number
+            +}
+            +
+            +export const SESSION_TTL = 3600
             """)
     }
 
@@ -648,11 +659,17 @@ final class RemoteStore: ObservableObject {
         ApprovalID(rawValue: "demo-appr-\(sessionID.rawValue)")
     }
 
-    func closeOpen() {
-        if let id = openSessionID {
-            send(.unsubscribe(id))
-            markOpened(id)  // everything up to now has been seen
-        }
+    /// Close a session's live subscription. `id` names *which* session is closing — the detail
+    /// view passes its own. On iPad's split view, switching session A→B can mount B (which calls
+    /// `open(B)`, setting `openSessionID = B`) *before* A's detail disappears; so we always
+    /// unsubscribe the named session but only tear down the shared open-state when it still
+    /// belongs to that session — otherwise we'd wipe B's freshly-loaded transcript. Called with
+    /// no argument it closes whatever is currently open (the iPhone push/pop path, unchanged).
+    func closeOpen(_ id: SessionID? = nil) {
+        guard let target = id ?? openSessionID else { return }
+        send(.unsubscribe(target))
+        markOpened(target)  // everything up to now has been seen
+        guard openSessionID == target else { return }
         openSessionID = nil
         openEvents = []
         openApprovals = []