Add iOS app with CarPlay and Siri

SwiftUI port of the Android app under ios/, with CarPlay browse and
Now Playing, Siri play-media intents, and a build-ipa.sh script that
makes an unsigned IPA for sideloading. README gets an iOS section and
the vibecoded disclaimer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/Resources/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -0,0 +1 @@
+{"images":[{"filename":"icon.png","idiom":"universal","platform":"ios","size":"1024x1024"}],"info":{"author":"xcode","version":1}}