Add Done button to grammar exercise score screen

Fixes stuck state after completing grammar exercises — adds a
dismiss button on the results screen.

Closes #14

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

Conjuga/Conjuga/Views/Guide/GrammarExerciseView.swift

@@ -3,6 +3,7 @@ import SwiftUI
 struct GrammarExerciseView: View {
     let noteId: String
     let noteTitle: String
+    @Environment(\.dismiss) private var dismiss
 
     @State private var exercises: [GrammarExercise] = []
     @State private var currentIndex = 0
@@ -122,6 +123,19 @@ struct GrammarExerciseView: View {
             Text(correctCount == exercises.count ? "Perfect!" : "Keep reviewing this topic.")
                 .font(.title3)
                 .foregroundStyle(.secondary)
+
+            Button {
+                dismiss()
+            } label: {
+                Text("Done")
+                    .font(.headline)
+                    .frame(maxWidth: .infinity)
+                    .padding(.vertical, 12)
+            }
+            .buttonStyle(.borderedProminent)
+            .tint(.purple)
+            .padding(.horizontal)
+
             Spacer()
         }
     }