fix: restore in-app back navigation under predictive back

MainActivity overrode onBackPressed() to return to the home view from the
settings and info screens. Apps targeting SDK 36 can no longer opt out of
predictive back, and the platform now routes back gestures through
OnBackPressedDispatcher instead of calling onBackPressed(), so back was
leaving the app instead of returning home. Lint flags this as
GestureBackNavigation, and only at targetSdk 36 -- the bump in #623 is what
introduced the regression.

Replace the override with an OnBackPressedCallback registered on the
dispatcher. The callback is enabled only while a non-home view is showing,
mirroring the existing up-arrow logic in chooseView(), so back still exits
the app from the home view. isShowingHomeView() is now unused and removed.

This needs androidx.activity 1.6+ to bridge the platform callback; the
pinned appcompat 1.4.0 only brings in activity 1.2.4. Upgrade appcompat to
1.7.1, which pulls activity 1.8.0 and still supports minSdk 21. Deliberately
not 1.8.0, which would require raising minSdk to 23.

Author: fortuna

Android/app/build.gradle

@@ -118,7 +118,7 @@ dependencies {
     androidTestImplementation 'androidx.annotation:annotation:1.3.0'
     androidTestImplementation 'androidx.test:runner:1.4.0'
     // UI libraries.
-    implementation 'androidx.appcompat:appcompat:1.4.0'
+    implementation 'androidx.appcompat:appcompat:1.7.1'
     implementation 'com.google.android.material:material:1.4.0'
     implementation 'androidx.preference:preference:1.1.1'
     implementation 'androidx.recyclerview:recyclerview:1.2.1'

Android/app/src/main/java/app/intra/ui/MainActivity.java

@@ -51,6 +51,7 @@
 import android.widget.Switch;
 import android.widget.TextView;
 import android.widget.Toast;
+import androidx.activity.OnBackPressedCallback;
 import androidx.annotation.DrawableRes;
 import androidx.annotation.IdRes;
 import androidx.annotation.StringRes;
@@ -177,6 +178,8 @@ protected void onCreate(Bundle savedInstanceState) {
 
     setContentView(R.layout.activity_main);
 
+    getOnBackPressedDispatcher().addCallback(this, backToHomeCallback);
+
     // Set up the toolbar
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
     setSupportActionBar(toolbar);
@@ -722,20 +725,23 @@ private void syncDnsStatus() {
       }
   }
 
-  @Override
-  public void onBackPressed() {
-    if (isShowingHomeView()) {
-      // Back button should leave the app if we are currently looking at the home view.
-      super.onBackPressed();
-      return;
+  /**
+   * Handles the back button on the settings and info views by returning to the home view.
+   *
+   * <p>This must not be an {@code onBackPressed()} override.  Apps targeting SDK 36 can no
+   * longer opt out of predictive back, and the platform routes back gestures through
+   * {@link androidx.activity.OnBackPressedDispatcher} instead of calling
+   * {@code onBackPressed()}.
+   *
+   * <p>The callback stays disabled while the home view is showing, so that back exits the
+   * app as usual.  {@link #chooseView} keeps that state in sync.
+   */
+  private final OnBackPressedCallback backToHomeCallback = new OnBackPressedCallback(false) {
+    @Override
+    public void handleOnBackPressed() {
+      chooseView(R.id.frame_main);
     }
-    chooseView(R.id.frame_main);
-  }
-
-  private boolean isShowingHomeView() {
-    View home = findViewById(R.id.frame_main);
-    return home.getVisibility() == View.VISIBLE;
-  }
+  };
 
   private void showSettings() {
     settingsFragment = new SettingsFragment();
@@ -781,6 +787,10 @@ private View chooseView(int id) {
       drawerToggle.setDrawerIndicatorEnabled(false);
       actionBar.setDisplayHomeAsUpEnabled(true);
     }
+
+    // Intercept back only on the screens the arrow points away from.  On the home view the
+    // callback is disabled, so back falls through to the system and leaves the app.
+    backToHomeCallback.setEnabled(id != R.id.frame_main);
     return selected;
   }