Add lead workflows, PDF signing, AR projects and photo gallery integration

GasTankAR/Services/AuthenticationService.swift

@@ -0,0 +1,22 @@
+import Foundation
+
+/// Domain boundary; the concrete server integration will replace the stub.
+protocol AuthenticationService: Sendable {
+    func signIn(salesCode: String) async throws -> LoginSession
+}
+
+struct LoginSession: Equatable, Sendable {
+    let salesCode: String
+    let displayName: String?
+
+    init(salesCode: String, displayName: String? = nil) {
+        self.salesCode = salesCode
+        self.displayName = displayName
+    }
+}
+
+enum LoginError: Error, Equatable {
+    case invalidSalesCode
+    case rejected
+    case unavailable
+}