Feature request: OAuth support for AI provider authentication

[barronlroth]

Feature Request

Currently Chat++ requires users to manually create and paste API keys from OpenAI, Anthropic, Google, xAI, etc. into a .env file. This works, but it's a friction point — especially for non-technical users who may not know how to generate API keys or manage env files.

Proposal

Support OAuth-based authentication for AI providers that offer it:

• Anthropic — OAuth via console.anthropic.com (supports OAuth 2.0 for third-party apps)
• OpenAI — OAuth via platform.openai.com
• Google AI — Google OAuth with Gemini API scopes

This would let users click "Sign in with Anthropic/OpenAI/Google" in the app UI instead of hunting for API keys. The app would handle the OAuth flow and store tokens securely in the macOS Keychain (via Tauri's secure storage).

Benefits

• Lower barrier to entry — no need to find, copy, and paste API keys
• Better security — tokens stored in Keychain, not plaintext .env files
• Token refresh — OAuth tokens can auto-refresh without manual intervention
• Easier multi-provider setup — connect all providers from a settings UI

Suggested UX

A Settings/Accounts panel in the app with "Connect" buttons per provider. Each triggers a browser-based OAuth flow that returns to the app with credentials. Providers without OAuth support can fall back to the existing API key input.

This is especially relevant as the app adds more AI features (timeline indexing, media captioning, assistant chat). Making auth seamless would be a great quality-of-life improvement.