fix(settings): force detail pane teardown when opening/closing Settings

Opening Settings while a document was pushed left the document
visible in the detail pane even though the sidebar correctly switched
to the settings list — NavigationSplitView on macOS doesn't reliably
replace the detail pane's content on an implicit branch change alone
between very different subtrees (a NavigationStack with a real push
history vs. a plain view). An explicit .id() keyed to
isShowingSettings forces a real teardown/remount; documentPath itself
is untouched, so returning via Done still lands back on the same
document.

Outpost/Features/Collections/ContentView_macOS.swift

@@ -66,6 +66,14 @@ struct ContentView_macOS: View {
             .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,