fix: Handle empty GHUI_KEY_ALIAS environment variable

- Check if GHUI_KEY_ALIAS is empty string and default to 'ghui'
- Prevents 'Keystore was tampered with' errors when key alias is empty
- Ensures consistent key alias handling in CI/CD

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: graycreate

app/build.gradle

@@ -34,7 +34,7 @@ android {
                 // This prevents the build from failing during configuration phase
                 storeFile file("keystore.jks")
                 storePassword System.getenv("GHUI_KEYSTORE_PASSWORD")
-                keyAlias System.getenv("GHUI_KEY_ALIAS") ?: "ghui"
+                keyAlias (System.getenv("GHUI_KEY_ALIAS") ?: "").trim().isEmpty() ? "ghui" : System.getenv("GHUI_KEY_ALIAS")
                 keyPassword System.getenv("GHUI_KEY_PASSWORD")
             } else {
                 // For local builds without proper signing setup, throw an error