fix(android): expose namespace to Flutter tooling

[SebastianBoehler]

Summary

• use the standard Android Gradle android {} block
• preserve the existing namespace, application ID, signing, SDK, and dependency configuration
• allow flutter run to detect com.studyostue.app before building

Root cause

Flutter resolves the package identifier from the manifest package or from a namespace inside an android {} block. The project used extensions.configure<ApplicationExtension> {}, which Gradle accepts but Flutter does not parse, so flutter run stopped with package identifier or launch activity not found.

Verification

• confirmed the launcher activity and MAIN/LAUNCHER intent are valid
• confirmed namespace and application ID remain com.studyostue.app
• confirmed the Gradle file now matches Flutter namespace detection
• git diff --check
• full Flutter/Android build delegated to PR CI because Flutter is not installed in the gateway image