Use modern OnBackPressedCallback instead of deprecated onBackPressed

Co-authored-by: TomTasche <128734+TomTasche@users.noreply.github.com>

Author: Copilot

app/src/main/java/at/tomtasche/reader/ui/activity/MainActivity.java

@@ -28,6 +28,7 @@
 import android.widget.CompoundButton;
 import android.widget.LinearLayout;
 
+import androidx.activity.OnBackPressedCallback;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.VisibleForTesting;
@@ -196,6 +197,26 @@ public void onClick(View view) {
         }
 
         addMenuProvider(this, this);
+
+        // Set up modern back button handling
+        getOnBackPressedDispatcher().addCallback(this, new OnBackPressedCallback(true) {
+            @Override
+            public void handleOnBackPressed() {
+                if (fullscreen) {
+                    leaveFullscreen();
+                    return;
+                }
+
+                // If document is currently displayed and was opened internally, go back to landing
+                if (documentContainer.getVisibility() == View.VISIBLE && documentOpenedInternally) {
+                    showLandingScreen();
+                } else {
+                    // For external launches or when already on landing screen, use default behavior (close app)
+                    setEnabled(false);
+                    getOnBackPressedDispatcher().onBackPressed();
+                }
+            }
+        });
     }
 
     @Override
@@ -572,22 +593,6 @@ public boolean onKeyDown(int keyCode, KeyEvent event) {
         return super.onKeyDown(keyCode, event);
     }
 
-    @Override
-    public void onBackPressed() {
-        if (fullscreen) {
-            leaveFullscreen();
-            return;
-        }
-
-        // If document is currently displayed and was opened internally, go back to landing
-        if (documentContainer.getVisibility() == View.VISIBLE && documentOpenedInternally) {
-            showLandingScreen();
-        } else {
-            // For external launches or when already on landing screen, use default behavior (close app)
-            super.onBackPressed();
-        }
-    }
-
     public void findDocument() {
         final Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
         intent.addCategory(Intent.CATEGORY_OPENABLE);