fix: use backend-authoritative change amount for cash transactions

Instead of calculating change client-side (which doesn't account for
balance applied, fees, etc.), the app now uses the Change value returned
by the /tasks/complete.php endpoint after processing.

Changes:
- APIService.completeTask now returns CashCompletionResult with backend values
- Added CashCompletionResult struct (cashReceived, orderTotal, change, fees, routing)
- CashCollectionSheet shows confirmed backend change after completion
- Added ratingRequired error case to APIError enum
- Client-side estimate still shown as preview before confirmation

PayfritWorks/Services/APIService.swift

@@ -56,6 +56,22 @@ struct ChatMessagesResult {
     let chatClosed: Bool
 }
 
+/// Backend-authoritative cash completion result.
+/// All monetary values are formatted strings (e.g. "12.50") from the server.
+struct CashCompletionResult {
+    let cashReceived: String
+    let orderTotal: String
+    let change: String
+    let customerFee: String?
+    let businessFee: String?
+    let cashRoutedTo: String  // "worker" or "business"
+    let balanceApplied: String?
+
+    var changeDollars: Double {
+        Double(change) ?? 0
+    }
+}
+
 // MARK: - API Service
 
 actor APIService {
@@ -474,7 +490,9 @@ actor APIService {
         return arr.map { WorkTask(json: $0) }
     }
 
-    func completeTask(taskId: Int, workerRating: [String: Bool]? = nil, cashReceivedCents: Int? = nil, cancelOrder: Bool = false) async throws {
+    /// Complete a task. Returns CashCompletionResult when cashReceivedCents is provided.
+    @discardableResult
+    func completeTask(taskId: Int, workerRating: [String: Bool]? = nil, cashReceivedCents: Int? = nil, cancelOrder: Bool = false) async throws -> CashCompletionResult? {
         var payload: [String: Any] = [
             "TaskID": taskId,
             "UserID": userId ?? 0
@@ -497,6 +515,20 @@ actor APIService {
             }
             throw APIError.serverError("Failed to complete task: \(errorMsg)")
         }
+
+        // Parse cash completion response from backend (authoritative values)
+        if let _ = cashReceivedCents, json["CashProcessed"] as? Bool == true {
+            return CashCompletionResult(
+                cashReceived: json["CashReceived"] as? String ?? "0.00",
+                orderTotal: json["OrderTotal"] as? String ?? "0.00",
+                change: json["Change"] as? String ?? "0.00",
+                customerFee: json["CustomerFee"] as? String,
+                businessFee: json["BusinessFee"] as? String,
+                cashRoutedTo: json["CashRoutedTo"] as? String ?? "worker",
+                balanceApplied: json["BalanceApplied"] as? String
+            )
+        }
+        return nil
     }
 
     func closeChat(taskId: Int) async throws {

PayfritWorks/Views/TaskDetailScreen.swift

@@ -834,6 +834,7 @@ struct CashCollectionSheet: View {
     @State private var cashReceivedText = ""
     @State private var isProcessing = false
     @State private var errorMessage: String?
+    @State private var completionResult: CashCompletionResult?
 
     private var isAdmin: Bool { roleId >= 2 }
 
@@ -844,7 +845,8 @@ struct CashCollectionSheet: View {
         return Int(round(dollars * 100))
     }
 
-    private var changeDue: Double? {
+    /// Client-side estimate shown before confirmation (preview only)
+    private var estimatedChangeDue: Double? {
         guard let receivedCents = cashReceivedCents else { return nil }
         let change = Double(receivedCents - orderTotalCents) / 100
         return change >= 0 ? change : nil
@@ -888,8 +890,35 @@ struct CashCollectionSheet: View {
                     .cornerRadius(12)
                 }
 
-                // Change display
+                // Change display — backend-authoritative after completion, estimate before
-                if let change = changeDue {
+                if let result = completionResult {
+                    // Show confirmed change from backend
+                    VStack(spacing: 8) {
+                        HStack {
+                            Image(systemName: "checkmark.circle.fill")
+                                .foregroundColor(.payfritGreen)
+                            Text("Change Due:")
+                                .font(.body.weight(.medium))
+                            Spacer()
+                            Text("$\(result.change)")
+                                .font(.title3.bold())
+                                .foregroundColor(.payfritGreen)
+                        }
+                        if let balanceApplied = result.balanceApplied {
+                            HStack {
+                                Text("Balance applied:")
+                                    .font(.caption)
+                                    .foregroundColor(.secondary)
+                                Spacer()
+                                Text("$\(balanceApplied)")
+                                    .font(.caption.weight(.medium))
+                            }
+                        }
+                    }
+                    .padding(16)
+                    .background(Color.payfritGreen.opacity(0.15))
+                    .cornerRadius(12)
+                } else if let change = estimatedChangeDue {
                     HStack {
                         Image(systemName: "arrow.uturn.left.circle.fill")
                             .foregroundColor(.payfritGreen)
@@ -954,7 +983,7 @@ struct CashCollectionSheet: View {
                 }
                 .buttonStyle(.borderedProminent)
                 .tint(Color(red: 0.13, green: 0.55, blue: 0.13))
-                .disabled(!isValid || isProcessing)
+                .disabled(!isValid || isProcessing || completionResult != nil)
                 .padding(.bottom, 16)
             }
             .padding(.horizontal, 24)
@@ -976,7 +1005,14 @@ struct CashCollectionSheet: View {
 
         Task {
             do {
-                try await APIService.shared.completeTask(taskId: taskId, cashReceivedCents: cents)
+                let result = try await APIService.shared.completeTask(taskId: taskId, cashReceivedCents: cents)
+                if let result = result {
+                    // Show backend-authoritative change before dismissing
+                    completionResult = result
+                    isProcessing = false
+                    // Brief pause so worker can see the confirmed change amount
+                    try? await Task.sleep(nanoseconds: 2_000_000_000)
+                }
                 dismiss()
                 onComplete()
             } catch {