Fix 5 build issues: add ButtonStyles to project, actor isolation, buffer mutability

- Re-ran xcodegen so ButtonStyles.swift is included (fixes PrimaryButtonStyle
  and ScaleButtonStyle not in scope in LoginView and LANTriggerSection)
- BackgroundTaskManager: access BackupEngine.shared inside @MainActor Task
  instead of nonisolated context
- SFTPService: change var buffer → let (never mutated)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Services/BackgroundTaskManager.swift

@@ -47,9 +47,8 @@ final class BackgroundTaskManager {
 
     private static func handleBackupTask(_ task: BGProcessingTask) {
         scheduleBackup()
-        let engine = BackupEngine.shared
-        task.expirationHandler = { Task { await engine.cancel() } }
-        Task {
+        task.expirationHandler = { Task { await BackupEngine.shared.cancel() } }
+        Task { @MainActor in
             do {
                 let store = ConnectionStore.shared
                 guard let connection = store.savedConnection else {
@@ -57,7 +56,7 @@ final class BackgroundTaskManager {
                     return
                 }
                 let filter = store.backupFilter
-                _ = try await engine.run(connection: connection, filter: filter, triggeredByLAN: true)
+                _ = try await BackupEngine.shared.run(connection: connection, filter: filter, triggeredByLAN: true)
                 task.setTaskCompleted(success: true)
             } catch {
                 task.setTaskCompleted(success: false)