Video circles: add on-device transcription

[mvsrcdst]

Closes #144

Summary

• Adds an on-device transcription for video circle messages, matching the existing behavior for voice messages
• Extracts the shared fetch/convert/transcribe signal chain into SGLocalTranscription so both message types reuse the same code instead of duplicating it
• Also fixes a bug: requiresOnDeviceRecognition was set from
  speechRecognizer.supportsOnDeviceRecognition, but that property
  always reads back true no matter what you assign it. So on-device
  recognition was forced for every locale. If the on-device model
  wasn't downloaded, transcription just failed instead of falling back
  to network recognition.

Screenshots

Video-circle message with transcribe button

Video-circle message with transcribed text

Testing

• Built and ran on iOS 18.6/iOS 26.5 Simulators
• Verified video circle transcribe button appears for non-Premium accounts
• Verified transcription produces text
• Verified via logging that on-device recognition is used when the locale's model is downloaded (requiresOnDeviceRecognition: true), confirming genuine offline operation
• Verified voice-message transcription (which shares the same fixed code path) still works correctly