Add multi-provider support with model selection and custom model input

[xROBERTINOx]

Summary

• Added provider selection (Groq, OpenAI, Together AI, Fireworks, NVIDIA NIM, Custom) to both the setup flow and settings
• Added model selection for both transcription and chat/post-processing models, with descriptions for each option
• Added "Custom" model option for every provider, allowing users to enter any model ID
• API key step in setup dynamically adapts to the selected provider (instructions, links, placeholder text)
• Per-provider API key storage so switching providers preserves previously entered keys

Inspired by #36 — rebuilt from scratch on top of the current main branch to avoid merge conflicts.

Setup Flow

Provider Selection:

Provider-Aware API Key:

Model Selection with Descriptions:

Settings

Provider & Model Settings:

Files Changed

• New: Sources/APIProvider.swift — Provider enum with model lists, descriptions, URLs, and custom model support
• Sources/SetupView.swift — Added provider and model selection steps to setup wizard
• Sources/AppState.swift — Added custom model ID storage, effective model ID resolution, per-provider key storage
• Sources/SettingsView.swift — Updated provider picker (dropdown) and model section (radio buttons with descriptions)
• Sources/TranscriptionService.swift — Accepts model as constructor parameter
• Sources/PostProcessingService.swift — Accepts model as constructor parameter
• Sources/AppContextService.swift — Accepts model as constructor parameter

[zachlatta]

Can you make a video showing this full new process working and successfully transcribing audio?

[zachlatta]

Thanks for this awesome PR! Excited to see your video

[fif911]

@zachlatta shall we get this merged? I think everything is working good. The video could be a bit of overhead, but if one spins the branch locally - it's possible to test for regressions and see that everything is good

[zachlatta]

Yes, I'm eager to get this merged! I'd like @xROBERTINOx to make a video (or someone else) to make a video showing the full thing works because this is a big PR.