fix(settings): make Settings its own top-level branch, not swapped content

The .id() fix on the detail pane wasn't enough — still reported as
the document staying visible with only the sidebar switching.
NavigationSplitView bridges to NSSplitViewController on macOS, and
apparently doesn't reliably replace already-mounted detail content
(a NavigationStack with real push history) for something unrelated
inside one persisting split view instance, identity hints or not.

Restructured so `navigation.isShowingSettings` picks between two
entirely separate NavigationSplitView instances (settingsContent /
mainContent) at the top of body, instead of branching on content
inside a single one. A different top-level view hierarchy is a
guaranteed full teardown of whatever AppKit was holding onto — no
split-view instance persists across the switch for it to get
confused about reusing.

Also dropped the now-redundant `isShowingSettings` guards scattered
through mainContent's toolbar/leadingToolbarContent — moot once
mainContent only ever renders while Settings isn't showing.

Outpost/Features/Collections/ContentView_macOS.swift

@@ -40,14 +40,51 @@ struct ContentView_macOS: View {
     }
 
     var body: some View {
-        NavigationSplitView {
-            Group {
+        // A totally separate top-level branch, not content swapped inside
+        // one persistent `NavigationSplitView` — that was tried first (an
+        // `if/else` inside a single split view, then an explicit `.id()` on
+        // just the detail pane) and neither reliably replaced the detail
+        // pane's content on macOS: `NavigationSplitView` bridges to
+        // `NSSplitViewController`, and swapping a `NavigationStack` with
+        // real push history for a plain view inside one persisting instance
+        // doesn't propagate the way plain SwiftUI identity rules would
+        // suggest. A different `if` branch is a genuinely different view
+        // hierarchy, so there's no existing split view instance for AppKit
+        // to get confused about reusing.
         if navigation.isShowingSettings {
+            settingsContent
+        } else {
+            mainContent
+        }
+    }
+
+    private var settingsContent: some View {
+        NavigationSplitView {
             SettingsSidebarList(
                 selection: selectedSettingsSectionBinding,
                 onDone: { navigation.isShowingSettings = false }
             )
-                } else {
+            .navigationSplitViewColumnWidth(min: 220, ideal: 260)
+        } detail: {
+            SettingsView(section: navigation.selectedSettingsSection ?? .appearance)
+        }
+        .navigationTitle("")
+        .toolbar {
+            ToolbarItem(placement: .navigation) {
+                HStack(spacing: 6) {
+                    Image(systemName: "gearshape.fill")
+                    Text("Settings")
+                }
+                .font(.headline)
+                .padding(.horizontal, 10)
+                .padding(.vertical, 4)
+                .background(.fill.tertiary, in: Capsule())
+            }
+        }
+    }
+
+    private var mainContent: some View {
+        NavigationSplitView {
             VStack(spacing: 0) {
                 SidebarSearchField(text: $globalSearchQuery)
                 Divider()
@@ -61,19 +98,9 @@ struct ContentView_macOS: View {
                 sidebar
                 AccountFooter()
             }
-                }
-            }
             .navigationSplitViewColumnWidth(min: 220, ideal: 260)
         } detail: {
             detail
-                // NavigationSplitView on macOS doesn't reliably tear down the
-                // detail pane's previous content when swapping between very
-                // different subtrees (a NavigationStack with a document
-                // pushed vs. plain SettingsView) off an implicit branch
-                // alone — an explicit identity change forces a real
-                // teardown/remount instead of it silently leaving the old
-                // document visible underneath.
-                .id(navigation.isShowingSettings)
         }
         // Empty rather than a real value: with one, the native title rendered
         // in the same toolbar row as the custom `.navigation` pill below,
@@ -108,7 +135,7 @@ struct ContentView_macOS: View {
             // `CollectionOverviewView`, so on Home it was a dead end: a
             // user could click it, type, and nothing would happen. The
             // sidebar's global search already covers "search everything."
-            if !navigation.isShowingSettings && !(isShowingHome && documentPath.isEmpty) {
+            if !(isShowingHome && documentPath.isEmpty) {
                 ToolbarItem(placement: .primaryAction) {
                     contextualSearchField
                 }
@@ -178,12 +205,7 @@ struct ContentView_macOS: View {
     @ViewBuilder
     private var leadingToolbarContent: some View {
         Group {
-            if navigation.isShowingSettings {
-                HStack(spacing: 6) {
-                    Image(systemName: "gearshape.fill")
-                    Text("Settings")
-                }
-            } else if !trimmedGlobalQuery.isEmpty {
+            if !trimmedGlobalQuery.isEmpty {
                 HStack(spacing: 6) {
                     Image(systemName: "magnifyingglass")
                     Text("Search")
@@ -262,7 +284,6 @@ struct ContentView_macOS: View {
         globalSearchQuery = ""
         selectedCollection = collection
         isContextualSearchExpanded = true
-        navigation.isShowingSettings = false
         Task { @MainActor in
             isContextualSearchFocused = true
         }
@@ -273,13 +294,11 @@ struct ContentView_macOS: View {
     /// hierarchy immediately rather than just the leaf.
     private func selectDocumentChain(_ collection: OutlineCollection, _ chain: [OutlineDocument]) {
         selectedCollection = collection
-        navigation.isShowingSettings = false
         replaceDocumentPath(with: chain)
     }
 
     /// Flat-list / search-result clicks: no known ancestors, single-level push.
     private func openDocument(_ document: OutlineDocument) {
-        navigation.isShowingSettings = false
         replaceDocumentPath(with: [document])
     }
 
@@ -298,14 +317,7 @@ struct ContentView_macOS: View {
 
     @ViewBuilder
     private var detail: some View {
-        if navigation.isShowingSettings {
-            // Deliberately not a `NavigationStack` destination or a separate
-            // window — it's swapped into the same detail pane Home/
-            // collections use, alongside the real sidebar (now showing
-            // `SettingsSidebarList` instead of the collections tree) rather
-            // than a page with its own nested mini sidebar.
-            SettingsView(section: navigation.selectedSettingsSection ?? .appearance)
-        } else if let apiClient = session.apiClient {
+        if let apiClient = session.apiClient {
             // A fresh NavigationStack per collection (or when entering/leaving
             // search), so switching either also clears any pushed document.
             NavigationStack(path: $documentPath) {