feat(transcription): preserve the detected source language

[jiezhengj]

Splits the source-language transcription work out of #80 for focused review (2/4).

Summary

• default the transcript-language preference to automatic detection
• explicitly use Whisper's transcribe task so non-English audio is not translated to English
• probe Whisper language tokens when no language hint is selected, then reuse the detected language for transcription
• persist the preference in saved projects and expose Auto-detect in the source-language selector
• keep Parakeet language detection automatic while using an explicit preference only for alignment

Scope

This PR is independent of UI localization, desktop window persistence, and runtime-asset packaging.

Validation

• npm run test:transcription
• npx tsc --noEmit
• npm run typecheck:electron
• npm run lint

[vercel]

@jiezhengj is attempting to deploy a commit to the wassgha's projects Team on Vercel.

A member of the Team first needs to authorize it.