fix: ios xcodegen — exclude .a from Externals scan to stop libapp.a being bundled as a Resource

App Store Connect rejected uploads with error 90171, "Peek Save.app/libapp.a binary file is not permitted." xcodegen recursively walks sources directories and assigns unrecognized extensions like .a to PBXResourcesBuildPhase by default — libapp.a was therefore added both to PBXFrameworksBuildPhase, which correctly links the static lib into the executable, and PBXResourcesBuildPhase, which copies it into the .app bundle as a standalone file. Excluding *.a from the Externals scan keeps the lib visible only via the existing dependencies framework declaration. After the next tauri ios build regenerates the pbxproj from this updated project.yml, the duplicate Resources entry disappears.