Persist sidebar visibility; add language-matching rule to default prompt

- ContentView now reads/writes SettingsService.sidebarVisible so the
  NavigationSplitView sidebar's shown/hidden state survives relaunch,
  matching the existing window size/position persistence.
- Default system prompt gains a rule: always reply in the user's
  language, even for requests (e.g. translation) targeting another one.

oAI/Views/Main/ContentView.swift

@@ -29,7 +29,8 @@ import Darwin   // uname, sysctlbyname
 struct ContentView: View {
     @Environment(ChatViewModel.self) var chatViewModel
     private var updateService = UpdateCheckService.shared
-    @State private var columnVisibility: NavigationSplitViewVisibility = .all
+    @State private var columnVisibility: NavigationSplitViewVisibility =
+        SettingsService.shared.sidebarVisible ? .all : .detailOnly
     @State private var showIntelWarning = false
 
     var body: some View {
@@ -46,6 +47,9 @@ struct ContentView: View {
             )
         }
         .frame(minWidth: 860, minHeight: 560)
+        .onChange(of: columnVisibility) { _, newValue in
+            SettingsService.shared.sidebarVisible = newValue != .detailOnly
+        }
         #if os(macOS)
         .onAppear {
             NSApplication.shared.windows.forEach { $0.tabbingMode = .disallowed }