Require PIN to disable Face ID

When toggling Face ID off, users must now verify their PIN first.
Adds a PinVerifyView component for PIN verification.

Fixes #23

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

PortfolioJournal/Views/Settings/SettingsView.swift

@@ -18,6 +18,7 @@ struct SettingsView: View {
     @State private var showingBiometricAlert = false
     @State private var showingPinRequiredAlert = false
     @State private var showingPinDisableAlert = false
+    @State private var showingPinVerifyForFaceId = false
     @State private var showingRestartAlert = false
     @State private var didLoadCloudSync = false
     @State private var backupToRestore: BackupRecord?
@@ -163,6 +164,13 @@ struct SettingsView: View {
                     _ = KeychainService.savePin(pin)
                 }
             }
+            .sheet(isPresented: $showingPinVerifyForFaceId) {
+                PinVerifyView(title: "Enter PIN to Disable Face ID") { success in
+                    if success {
+                        faceIdEnabled = false
+                    }
+                }
+            }
             .onAppear {
                 didLoadCloudSync = true
                 if viewModel.backupsEnabled {
@@ -495,6 +503,9 @@ struct SettingsView: View {
                             faceIdEnabled = false
                             showingPinRequiredAlert = true
                         }
+                    } else {
+                        faceIdEnabled = true
+                        showingPinVerifyForFaceId = true
                     }
                 }
 
@@ -860,6 +871,62 @@ struct PinSetupView: View {
     }
 }
 
+struct PinVerifyView: View {
+    @Environment(\.dismiss) private var dismiss
+    let title: String
+    let onResult: (Bool) -> Void
+
+    @State private var pin = ""
+    @State private var errorMessage: String?
+
+    var body: some View {
+        NavigationStack {
+            Form {
+                Section {
+                    SecureField("Enter PIN", text: $pin)
+                        .keyboardType(.numberPad)
+                        .onChange(of: pin) { _, newValue in
+                            pin = String(newValue.filter(\.isNumber).prefix(4))
+                        }
+                } header: {
+                    Text("4-Digit PIN")
+                }
+
+                if let errorMessage {
+                    Text(errorMessage)
+                        .font(.caption)
+                        .foregroundColor(.negativeRed)
+                }
+            }
+            .navigationTitle(title)
+            .navigationBarTitleDisplayMode(.inline)
+            .toolbar {
+                ToolbarItem(placement: .navigationBarLeading) {
+                    Button("Cancel") {
+                        onResult(false)
+                        dismiss()
+                    }
+                }
+                ToolbarItem(placement: .navigationBarTrailing) {
+                    Button("Verify") { verifyPin() }
+                        .disabled(pin.count < 4)
+                }
+            }
+        }
+        .presentationDetents([.medium])
+    }
+
+    private func verifyPin() {
+        guard let savedPin = KeychainService.readPin(), pin == savedPin else {
+            errorMessage = "Incorrect PIN."
+            pin = ""
+            return
+        }
+        onResult(true)
+        dismiss()
+    }
+}
+
 #Preview {
     SettingsView(iapService: IAPService())
         .environmentObject(AccountStore(iapService: IAPService()))