fix: copy Android JNI libs as libapp.a to match uniffi-generated CMakeLists

[portal-wheatley]

Problem

After the rearchitecture in #133 (crate renamed app → portal-app → reverted back to app), the react-native/flake.nix was updated to copy the Android JNI libraries under the wrong filename:

# before this fix
cp ${libAndroidAarch64}/lib/libapp.a android/src/main/jniLibs/arm64-v8a/libportal.a
cp ${libAndroidX86_64}/lib/libapp.a android/src/main/jniLibs/x86_64/libportal.a

uniffi-bindgen-react-native generates a CMakeLists.txt that references the library by the Rust crate name — which is app, so it looks for libapp.a. Since the flake was copying it as libportal.a, the Android build failed:

ninja: error: 'src/main/jniLibs/arm64-v8a/libapp.a', needed by 'libportal-app-lib.so', missing

Fix

Keep the destination filename consistent with the crate name:

cp ${libAndroidAarch64}/lib/libapp.a android/src/main/jniLibs/arm64-v8a/libapp.a
cp ${libAndroidX86_64}/lib/libapp.a android/src/main/jniLibs/x86_64/libapp.a