Add release signing, fix release build, update README

- Configure release signing via environment variables (android-signing-env)
- Enable BuildConfig generation and guard HttpLoggingInterceptor to debug only
- Move okhttp.logging to implementation so release build compiles
- Update README: add E2EE, verification, push notifications, download link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/build.gradle.kts

@@ -18,6 +18,16 @@ android {
         versionName = "0.1.0"
     }
 
+    signingConfigs {
+        create("release") {
+            storeFile = file(System.getenv("ANDROID_KEYSTORE_PATH") ?: "")
+            storeType = System.getenv("ANDROID_KEYSTORE_TYPE") ?: "PKCS12"
+            keyAlias = System.getenv("ANDROID_KEY_ALIAS") ?: ""
+            storePassword = System.getenv("ANDROID_KEYSTORE_PASSWORD") ?: ""
+            keyPassword = System.getenv("ANDROID_KEY_PASSWORD") ?: ""
+        }
+    }
+
     buildTypes {
         release {
             isMinifyEnabled = true
@@ -25,6 +35,7 @@ android {
                 getDefaultProguardFile("proguard-android-optimize.txt"),
                 "proguard-rules.pro"
             )
+            signingConfig = signingConfigs.getByName("release")
         }
     }
 
@@ -39,6 +50,7 @@ android {
 
     buildFeatures {
         compose = true
+        buildConfig = true
     }
 
     packaging {
@@ -68,5 +80,5 @@ dependencies {
     implementation(platform(libs.firebase.bom))
     implementation(libs.firebase.messaging)
     debugImplementation(libs.androidx.ui.tooling)
-    debugImplementation(libs.okhttp.logging)
+    implementation(libs.okhttp.logging)
 }