Language handling overhaul

[yujonglee]

• New language top-level key in settings.json.
  • feat(settings): store array fields as JSON instead of comma-separated strings #2838
• Use system-preferred language as default in macOS
  • feat(detect): add OS language detection and feature flags for crate #2784
• split language to variants #2827
  • Language format and conversion overhaul #2836
• Make sure we respect "spoken_languages` during transcribe.
  • App follows system language when transcribe? #2724
• Add validation/guidance for language/provider selection #2897

[yujonglee]

#2839