Update .gitignore and configure signing for release build in build.gradle.kts

.gitignore

@@ -1,3 +1,4 @@
 data/en.openfoodfacts.org.products.csv
 mobile/release-key.jks
 mobile/release-key-base64.txt
+mobile/src-tauri/gen/android/keystore.properties

mobile/src-tauri/gen/android/app/build.gradle.kts

@@ -1,4 +1,5 @@
 import java.util.Properties
+import java.io.FileInputStream
 
 plugins {
     id("com.android.application")
@@ -53,6 +54,7 @@ android {
             }
         }
         getByName("release") {
+			signingConfig = signingConfigs.getByName("release")
             isMinifyEnabled = true
             proguardFiles(
                 *fileTree(".") { include("**/*.pro") }