Initial scaffold — NASBackup iOS app

Full project scaffold: XcodeGen project.yml, Core models/protocols/errors,
SMBService (kishikawakatsumi/SMBClient), SFTPService (Citadel 0.9.2),
PhotoLibraryService, LANMonitor, BackupEngine, BackgroundTaskManager,
all feature UIs (Login, Connect, Browse, Backup, History, Settings),
Shared components and theme. Builds clean with zero errors.

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

Core/Errors/BackupError.swift

@@ -0,0 +1,27 @@
+import Foundation
+
+enum BackupError: LocalizedError {
+    case connectionFailed(String)
+    case authenticationFailed
+    case photoLibraryDenied
+    case uploadFailed(String, underlying: Error)
+    case directoryListFailed(String)
+    case directoryCreateFailed(String)
+    case cancelled
+    case timeout
+    case networkUnavailable
+
+    var errorDescription: String? {
+        switch self {
+        case .connectionFailed(let host):   return "Could not connect to \(host)"
+        case .authenticationFailed:         return "Authentication failed — check username and password"
+        case .photoLibraryDenied:           return "Photos access denied — update in iOS Settings"
+        case .uploadFailed(let file, _):    return "Failed to upload \(file)"
+        case .directoryListFailed(let path):return "Could not list \(path)"
+        case .directoryCreateFailed(let p): return "Could not create folder at \(p)"
+        case .cancelled:                    return "Backup was cancelled"
+        case .timeout:                      return "Operation timed out"
+        case .networkUnavailable:           return "Network not available"
+        }
+    }
+}