[codex] Add Sherpa ONNX ASR support

[hehehai]

Summary

• Add Sherpa ONNX ASR model support and runtime integration.
• Update local ASR model catalog, settings, and transcription routing for the new backend.
• Remove WhisperKit runtime paths and update related documentation/tests.

Impact

This changes the local ASR backend surface and model-management behavior for macOS users. It also refreshes related settings, onboarding, and documentation around local transcription models.

Validation

• Attempted: xcodebuild test -project Voxt.xcodeproj -scheme Voxt -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO.
• Result: interrupted after the command stayed in Resolve Package Graph / SwiftPM manifest resolution for about 6 minutes with xcodebuild and child manifest processes at 0% CPU.