feat(flow): add Picture in Picture keep-alive mode

- Add FlowKeepAliveMode (Dynamic Island vs PiP) with iCloud sync
- Settings: keep-alive picker; inactivity timeout only for Live Activity
- PiP: waveform sample-buffer controller, mic released between utterances
- PiP sessions have no idle expiry; user closing PiP ends the session
- FlowSessionManager branches hostReady, session start, and utterance paths
- UIBackgroundModes picture-in-picture; bilingual strings and tests

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

project.yml

@@ -133,6 +133,7 @@ targets:
         NSSpeechRecognitionUsageDescription: "OSGKeyboard uses speech recognition to transcribe your voice. Audio is processed on-device by default, or sent to your configured speech provider only when you enable cloud recognition."
         UIBackgroundModes:
           - audio
+          - picture-in-picture
         NSSupportsLiveActivities: true
         NSAppTransportSecurity:
           NSAllowsArbitraryLoads: false