feat(US-16): configure release signing and minification

Signing config reads from keystore.properties (gitignored).
Release build has minification enabled. The keystore and
properties file are excluded from version control.

To build a signed release APK:
  ./gradlew assembleRelease
Output: app/build/outputs/apk/release/app-release.apk

.gitignore

@@ -12,3 +12,8 @@ local.properties
 
 # Claude Code session data
 .claude/
+
+# Signing credentials — keep off version control
+*.jks
+*.keystore
+keystore.properties

app/build.gradle.kts

@@ -1,9 +1,16 @@
+import java.util.Properties
+
 plugins {
     alias(libs.plugins.android.application)
     alias(libs.plugins.kotlin.android)
     alias(libs.plugins.kotlin.compose)
 }
 
+val keystorePropsFile = rootProject.file("keystore.properties")
+val keystoreProps = Properties().apply {
+    if (keystorePropsFile.exists()) load(keystorePropsFile.inputStream())
+}
+
 android {
     namespace = "net.jeena.pacer"
     compileSdk = 36
@@ -19,9 +26,21 @@ android {
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }
 
+    signingConfigs {
+        create("release") {
+            if (keystorePropsFile.exists()) {
+                storeFile = file(keystoreProps["storeFile"] as String)
+                storePassword = keystoreProps["storePassword"] as String
+                keyAlias = keystoreProps["keyAlias"] as String
+                keyPassword = keystoreProps["keyPassword"] as String
+            }
+        }
+    }
+
     buildTypes {
         release {
-            isMinifyEnabled = false
+            isMinifyEnabled = true
+            signingConfig = signingConfigs.getByName("release")
             proguardFiles(
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"