Guard quiz Next/Previous against double taps

A rapid second tap on Next (easy to trigger with Apple Pencil) called
advance() twice before the view tree re-rendered, skipping a card.
Share a single isAdvancing flag between advance() and a new goBack()
helper, disable both buttons while the flag is set, and clear it after
350 ms so queued Pencil events are absorbed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Views/Course/CourseQuizView.swift

@@ -18,6 +18,7 @@ struct CourseQuizView: View {
     @State private var currentIndex = 0
     @State private var correctCount = 0
     @State private var missedItems: [MissedCourseItem] = []
+    @State private var isAdvancing = false
 
     // Per-question state
     @State private var userAnswer = ""
@@ -311,16 +312,13 @@ struct CourseQuizView: View {
             // Nav arrows
             HStack {
                 Button {
-                    guard currentIndex > 0 else { return }
-                    currentIndex -= 1
-                    resetQuestion()
-                    prepareQuestion()
+                    goBack()
                 } label: {
                     Label("Previous", systemImage: "chevron.left")
                         .font(.subheadline)
                 }
                 .tint(.secondary)
-                .disabled(currentIndex == 0)
+                .disabled(currentIndex == 0 || isAdvancing)
 
                 Spacer()
 
@@ -332,6 +330,7 @@ struct CourseQuizView: View {
                         .labelStyle(.titleAndIcon)
                 }
                 .tint(.blue)
+                .disabled(isAdvancing)
             }
             .padding(.horizontal)
         }
@@ -498,6 +497,8 @@ struct CourseQuizView: View {
     }
 
     private func advance() {
+        guard !isAdvancing else { return }
+        isAdvancing = true
         currentIndex += 1
         if isComplete {
             saveResult()
@@ -505,6 +506,20 @@ struct CourseQuizView: View {
             resetQuestion()
             prepareQuestion()
         }
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.35) {
+            isAdvancing = false
+        }
+    }
+
+    private func goBack() {
+        guard !isAdvancing, currentIndex > 0 else { return }
+        isAdvancing = true
+        currentIndex -= 1
+        resetQuestion()
+        prepareQuestion()
+        DispatchQueue.main.asyncAfter(deadline: .now() + 0.35) {
+            isAdvancing = false
+        }
     }
 
     private func saveResult() {