Add release signing configuration

- Load signing credentials from local.properties (not committed)
- Keystore stored in .signing/ directory (not committed)
- Release builds are now signed and installable

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/build.gradle.kts

@@ -1,13 +1,35 @@
+import java.util.Properties
+
 plugins {
     alias(libs.plugins.android.application)
     alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.compose)
 }
 
+// Load signing properties from local.properties
+val keystoreProperties = Properties().apply {
+    val localPropertiesFile = rootProject.file("local.properties")
+    if (localPropertiesFile.exists()) {
+        load(localPropertiesFile.inputStream())
+    }
+}
+
 android {
     namespace = "no.naiv.tiltshift"
     compileSdk = 35
 
+    signingConfigs {
+        create("release") {
+            val storeFilePath = keystoreProperties["storeFile"] as String?
+            if (storeFilePath != null) {
+                storeFile = file(storeFilePath)
+                storePassword = keystoreProperties["storePassword"] as String
+                keyAlias = keystoreProperties["keyAlias"] as String
+                keyPassword = keystoreProperties["keyPassword"] as String
+            }
+        }
+    }
+
     defaultConfig {
         applicationId = "no.naiv.tiltshift"
         minSdk = 35
@@ -26,6 +48,7 @@ android {
         release {
             isMinifyEnabled = true
             isShrinkResources = true
+            signingConfig = signingConfigs.getByName("release")
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"