fix: stabilize view tree for zoom transition recovery

ROOT CAUSE: @State TripsViewModel inside TripsHomeView was recreated
during navigation transitions. Each recreation set isLoading=true,
flipping the Group conditional, destroying the ScrollView + namespace.
The zoom transition's matchedTransitionSource evaporated.

FIXES (matching Apple's Wishlist pattern):
1. ViewModel owned by MainTabView (like ReaderVM) — survives
   view recreation. Pre-loaded on app launch.
2. Loading state rendered INSIDE ScrollView — no conditional
   Group wrapper that swaps the entire view tree.
3. Single @Namespace in TripsHomeView, passed to both
   UpcomingTripsPageView and PastTripsSection.
4. Zoom transition restored on all NavigationLinks.

Why Apple's sample works: they use @Environment(DataSource.self)
which is app-level stable. Our equivalent: @State at MainTabView.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/ContentView.swift

@@ -28,6 +28,7 @@ struct MainTabView: View {
     @State private var showAssistant = false
     @State private var confettiTrigger = 0
     @State private var readerVM = ReaderViewModel()
+    @State private var tripsVM = TripsViewModel()
     @State private var isAutoScrolling = false
     @State private var scrollSpeed: Double = 1.5
     @State private var speedLevel = 0  // 0=slow, 1=med, 2=fast
@@ -82,7 +83,7 @@ struct MainTabView: View {
                 }
 
                 Tab("Trips", systemImage: "airplane", value: 4) {
-                    TripsHomeView()
+                    TripsHomeView(vm: tripsVM)
                 }
 
                 // Action button — separated circle on trailing side of tab bar
@@ -137,6 +138,10 @@ struct MainTabView: View {
             }
         }
         .task {
+            guard showReader else { return }
+            // Pre-load trips for all users
+            await tripsVM.loadTrips()
+
             guard showReader else { return }
             let renderer = ArticleRenderer.shared
             renderer.attachToWindow()

ios/Platform/Platform/Features/Trips/Views/PastTripsSection.swift

@@ -4,6 +4,7 @@ import SwiftUI
 /// Adapted from Apple's Wishlist TripCollectionView.
 struct PastTripsSection: View {
     let trips: [Trip]
+    var namespace: Namespace.ID
 
     var body: some View {
         if !trips.isEmpty {
@@ -13,8 +14,11 @@ struct PastTripsSection: View {
                         ForEach(trips) { trip in
                             NavigationLink {
                                 TripPlaceholderView(trip: trip)
+                                    .navigationTransition(
+                                        .zoom(sourceID: trip.id, in: namespace))
                             } label: {
                                 TripCard(trip: trip, size: .compact)
+                                    .matchedTransitionSource(id: trip.id, in: namespace)
                                     .contentShape(.rect)
                             }
                             .buttonStyle(.plain)

ios/Platform/Platform/Features/Trips/Views/TripsHomeView.swift

@@ -2,29 +2,37 @@ import SwiftUI
 
 /// The main Trips home screen — inspired by Apple's Wishlist sample.
 ///
-/// Layout:
-/// 1. Hero section: upcoming trips as full-width paged cards
-/// 2. Secondary section: past trips in horizontal compact cards
-///
-/// Adapted from WishlistView structure: NavigationStack + ScrollView + sections.
+/// ViewModel owned by MainTabView (stable across navigation transitions).
+/// Always renders the ScrollView — no conditional loading/content flip
+/// that would destroy the namespace during transitions.
 struct TripsHomeView: View {
-    @State private var vm = TripsViewModel()
+    @Bindable var vm: TripsViewModel
+
+    @Namespace private var namespace
 
     var body: some View {
         NavigationStack {
-            Group {
-                if vm.isLoading && vm.trips.isEmpty {
-                    LoadingView()
-                        .frame(maxWidth: .infinity, maxHeight: .infinity)
-                } else {
             ScrollView {
+                if vm.isLoading && vm.trips.isEmpty {
+                    // Loading inside the ScrollView — keeps view tree stable
+                    VStack {
+                        Spacer(minLength: 200)
+                        ProgressView()
+                            .controlSize(.regular)
+                        Text("Loading trips...")
+                            .font(.caption)
+                            .foregroundStyle(Color.textTertiary)
+                            .padding(.top, 8)
+                        Spacer(minLength: 200)
+                    }
+                    .frame(maxWidth: .infinity)
+                } else {
                     VStack(alignment: .leading, spacing: 10) {
-                            // Hero: upcoming trips (paged, full-width)
-                            UpcomingTripsPageView(trips: vm.upcomingTrips)
+                        UpcomingTripsPageView(trips: vm.upcomingTrips, namespace: namespace)
                             .padding(.bottom, 20)
 
-                            // Secondary: past trips (horizontal scroll, compact cards)
-                            PastTripsSection(trips: vm.pastTrips)
+                        PastTripsSection(trips: vm.pastTrips, namespace: namespace)
+                    }
                 }
             }
             .contentMargins(.bottom, 30, for: .scrollContent)
@@ -32,11 +40,8 @@ struct TripsHomeView: View {
             .refreshable {
                 await vm.refresh()
             }
-                }
-            }
             .background(Color.canvas)
             .toolbar {
-                // Custom expanded title — Wishlist pattern
                 ToolbarItem(placement: .title) {
                     HStack {
                         Text("Trips")
@@ -48,7 +53,6 @@ struct TripsHomeView: View {
                     }
                 }
 
-                // Plan Trip button — glass prominent style
                 ToolbarItem(placement: .topBarTrailing) {
                     Button {
                         // Phase 2: present plan trip sheet
@@ -63,8 +67,5 @@ struct TripsHomeView: View {
             .toolbarTitleDisplayMode(.inline)
             .toolbarRole(.editor)
         }
-        .task {
-            await vm.loadTrips()
-        }
     }
 }

ios/Platform/Platform/Features/Trips/Views/UpcomingTripsPageView.swift

@@ -2,12 +2,15 @@ import SwiftUI
 
 /// Displays upcoming trips in a paged TabView — the hero section.
 /// Adapted from Apple's Wishlist RecentTripsPageView.
+///
+/// Namespace is passed from the parent (TripsHomeView) to ensure
+/// stability across navigation transitions.
 struct UpcomingTripsPageView: View {
     let trips: [Trip]
+    var namespace: Namespace.ID
 
     var body: some View {
         if trips.isEmpty {
-            // No upcoming trips — show a prompt
             VStack(spacing: 16) {
                 Image(systemName: "airplane.departure")
                     .font(.system(size: 44))
@@ -26,6 +29,8 @@ struct UpcomingTripsPageView: View {
                 ForEach(trips) { trip in
                     NavigationLink {
                         TripPlaceholderView(trip: trip)
+                            .navigationTransition(
+                                .zoom(sourceID: trip.id, in: namespace))
                     } label: {
                         TripImageView(url: trip.imageURL, fallbackName: trip.name)
                             .overlay(alignment: .bottomLeading) {
@@ -48,6 +53,7 @@ struct UpcomingTripsPageView: View {
                                 .padding(.horizontal)
                                 .padding(.bottom, 54)
                             }
+                            .matchedTransitionSource(id: trip.id, in: namespace)
                     }
                     .buttonStyle(.plain)
                 }