feat(collections): detect remote changes, offer a refresh banner

Every 45s, a background check fetches fresh collections/documents and
compares an id+updatedAt fingerprint against what's displayed — on a
mismatch, shows a "New changes available" banner instead of silently
swapping content out from under whoever's looking at it (which would
also lose sidebar scroll position / expanded rows). Tapping Refresh
does the actual reload.

Outpost/Features/Collections/CollectionOverviewView.swift

@@ -34,6 +34,12 @@ struct CollectionOverviewView: View {
             // No in-content header — the collection's icon/title live in the
             // window toolbar now (via `ContentView_macOS`), so this doesn't
             // duplicate it directly below.
+            if viewModel.hasRemoteChanges {
+                RemoteChangesBanner {
+                    Task { await viewModel.load() }
+                }
+            }
+
             if trimmedSearchQuery.isEmpty {
                 tabBar
             }
@@ -62,6 +68,13 @@ struct CollectionOverviewView: View {
             guard !Task.isCancelled else { return }
             await searchViewModel.search(query: trimmedSearchQuery)
         }
+        .task {
+            while !Task.isCancelled {
+                try? await Task.sleep(for: .seconds(45))
+                guard !Task.isCancelled else { break }
+                await viewModel.checkForRemoteChanges()
+            }
+        }
     }
 
     // Spans the full window width, centered, directly under the toolbar —

Outpost/Features/Collections/CollectionsTreeView.swift

@@ -25,6 +25,25 @@ struct CollectionsTreeView: View {
     }
 
     var body: some View {
+        VStack(spacing: 0) {
+            if viewModel.hasRemoteChanges {
+                RemoteChangesBanner {
+                    Task { await viewModel.load() }
+                }
+            }
+            content
+        }
+        .task {
+            while !Task.isCancelled {
+                try? await Task.sleep(for: .seconds(45))
+                guard !Task.isCancelled else { break }
+                await viewModel.checkForRemoteChanges()
+            }
+        }
+    }
+
+    @ViewBuilder
+    private var content: some View {
         Group {
             if viewModel.isLoading && viewModel.collections.isEmpty {
                 ProgressView()

Outpost/Features/Collections/CollectionsViewModel.swift

@@ -8,6 +8,7 @@ final class CollectionsViewModel {
     private(set) var collections: [OutlineCollection] = []
     var isLoading = false
     var errorMessage: String?
+    var hasRemoteChanges = false
 
     let apiClient: OutlineAPIClient
 
@@ -22,8 +23,27 @@ final class CollectionsViewModel {
 
         do {
             collections = try await apiClient.listCollections(offset: 0, limit: 100)
+            hasRemoteChanges = false
         } catch {
             errorMessage = "Couldn't load collections. Check your connection and try again."
         }
     }
+
+    /// Fetches fresh data to compare against what's displayed, without
+    /// replacing it — `hasRemoteChanges` drives a "Refresh" banner instead of
+    /// silently swapping content (and losing scroll position/expanded state)
+    /// out from under whoever's looking at it.
+    func checkForRemoteChanges() async {
+        guard let fresh = try? await apiClient.listCollections(offset: 0, limit: 100) else { return }
+        if Self.fingerprint(fresh) != Self.fingerprint(collections) {
+            hasRemoteChanges = true
+        }
+    }
+
+    private static func fingerprint(_ collections: [OutlineCollection]) -> String {
+        collections
+            .map { "\($0.id):\($0.updatedAt.timeIntervalSince1970)" }
+            .sorted()
+            .joined(separator: "|")
+    }
 }

Outpost/Features/Collections/DocumentsViewModel.swift

@@ -8,6 +8,7 @@ final class DocumentsViewModel {
     private(set) var documents: [OutlineDocument] = []
     var isLoading = false
     var errorMessage: String?
+    var hasRemoteChanges = false
 
     let collection: OutlineCollection
     private let apiClient: OutlineAPIClient
@@ -24,8 +25,26 @@ final class DocumentsViewModel {
 
         do {
             documents = try await apiClient.listDocuments(collectionId: collection.id, offset: 0, limit: 100)
+            hasRemoteChanges = false
         } catch {
             errorMessage = "Couldn't load documents. Check your connection and try again."
         }
     }
+
+    /// See CollectionsViewModel.checkForRemoteChanges — same reasoning.
+    func checkForRemoteChanges() async {
+        guard let fresh = try? await apiClient.listDocuments(collectionId: collection.id, offset: 0, limit: 100) else {
+            return
+        }
+        if Self.fingerprint(fresh) != Self.fingerprint(documents) {
+            hasRemoteChanges = true
+        }
+    }
+
+    private static func fingerprint(_ documents: [OutlineDocument]) -> String {
+        documents
+            .map { "\($0.id):\($0.updatedAt.timeIntervalSince1970)" }
+            .sorted()
+            .joined(separator: "|")
+    }
 }

Outpost/Features/Collections/RemoteChangesBanner.swift

@@ -0,0 +1,26 @@
+#if os(macOS)
+import SwiftUI
+
+/// Shown when a periodic background check finds the server has changes we
+/// don't have — doesn't auto-refresh, since that would silently replace
+/// what's on screen (losing scroll position, expanded rows) without asking.
+struct RemoteChangesBanner: View {
+    let onRefresh: () -> Void
+
+    var body: some View {
+        HStack(spacing: 8) {
+            Image(systemName: "arrow.triangle.2.circlepath")
+                .foregroundStyle(Color.accentColor)
+            Text("New changes available")
+                .font(.callout)
+            Spacer(minLength: 8)
+            Button("Refresh", action: onRefresh)
+                .buttonStyle(.borderedProminent)
+                .controlSize(.small)
+        }
+        .padding(.horizontal, 12)
+        .padding(.vertical, 8)
+        .background(Color.accentColor.opacity(0.12))
+    }
+}
+#endif