Externalize PUSH_GATEWAY_URL and clean up signing config

- Move PUSH_GATEWAY_URL from hardcoded constant to BuildConfig field,
  read from PUSH_GATEWAY_URL env var at build time
- Hardcode keyAlias="release" and remove storeType (PKCS12 is default),
  since these are fixed values in android-signing-env
- Remove unused okhttp3.Interceptor import

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

app/build.gradle.kts

@@ -16,13 +16,14 @@ android {
         targetSdk = 35
         versionCode = 1
         versionName = "0.1.0"
+        buildConfigField("String", "PUSH_GATEWAY_URL",
+            "\"${System.getenv("PUSH_GATEWAY_URL") ?: ""}\"")
     }
 
     signingConfigs {
         create("release") {
             storeFile = file(System.getenv("ANDROID_KEYSTORE_PATH") ?: "")
-            storeType = System.getenv("ANDROID_KEYSTORE_TYPE") ?: "PKCS12"
-            keyAlias = System.getenv("ANDROID_KEY_ALIAS") ?: ""
+            keyAlias = "release"
             storePassword = System.getenv("ANDROID_KEYSTORE_PASSWORD") ?: ""
             keyPassword = System.getenv("ANDROID_KEY_PASSWORD") ?: ""
         }