feat: turn Sst into a regular Mac app with settings, menus and menu bar item

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Sources/Sst/MenuBarContent.swift

@@ -0,0 +1,23 @@
+import SwiftUI
+
+/// The secondary interface: status and shortcut while working in another app.
+struct MenuBarContent: View {
+    @EnvironmentObject private var model: AppModel
+    @Environment(\.openWindow) private var openWindow
+
+    var body: some View {
+        Text(model.statusLine)
+        if let error = model.lastError {
+            Text(error)
+        }
+        Divider()
+        Button(Strings.openApp) {
+            openWindow(id: "main")
+            model.show(model.section)
+        }
+        SettingsLink { Text(Strings.settings) }
+        Divider()
+        Button(Strings.quit) { NSApplication.shared.terminate(nil) }
+            .keyboardShortcut("q")
+    }
+}