build(android): configure release signing with keystore

Replaces default debug signing with a dedicated release keystore loaded via android/key.properties. Fixes the cross-host signature mismatch that broke adb install -r. keystore and key.properties are gitignored (signing identity must not be committed).

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -46,6 +46,10 @@ app.*.map.json
 /android/app/release
 /android/build/
 
+# Release signing keystore (do NOT commit — contains signing identity)
+/android/key.properties
+/android/app/key.jks
+
 # AI Agents
 .claude/
 .agents/

android/app/build.gradle.kts

@@ -1,3 +1,6 @@
+import java.util.Properties
+import java.io.FileInputStream
+
 plugins {
     id("com.android.application")
     id("kotlin-android")
@@ -5,11 +8,26 @@ plugins {
     id("dev.flutter.flutter-gradle-plugin")
 }
 
+val keystoreProperties = Properties()
+val keystorePropertiesFile = rootProject.file("key.properties")
+if (keystorePropertiesFile.exists()) {
+    keystoreProperties.load(FileInputStream(keystorePropertiesFile))
+}
+
 android {
     namespace = "com.example.pad_scanner"
     compileSdk = flutter.compileSdkVersion
     ndkVersion = flutter.ndkVersion
 
+    signingConfigs {
+        create("release") {
+            keyAlias = keystoreProperties["keyAlias"] as String
+            keyPassword = keystoreProperties["keyPassword"] as String
+            storeFile = file(keystoreProperties["storeFile"] as String)
+            storePassword = keystoreProperties["storePassword"] as String
+        }
+    }
+
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_11
         targetCompatibility = JavaVersion.VERSION_11
@@ -32,9 +50,7 @@ android {
 
     buildTypes {
         release {
-            // TODO: Add your own signing config for the release build.
-            // Signing with the debug keys for now, so `flutter run --release` works.
-            signingConfig = signingConfigs.getByName("debug")
+            signingConfig = signingConfigs.getByName("release")
         }
     }
 }