Support Anthropics OAuth as new provider

[mikaelliljedahl]

Summary by CodeRabbit

• New Features

  • Provider system with Anthropic OAuth login (PKCE) and OpenRouter adapter, provider selection/settings UI, browser-based login dialogs, encrypted token storage, and provider-aware model selection.

• Refactor

  • Pluggable provider interfaces, unified streaming/non-streaming chat bridging, provider-aware startup/configuration, and UI updated to show/manage current provider and models.

• Tests

  • Expanded unit/integration tests covering providers, auth, token store, PKCE, message conversion, streaming, and provider factory.

• Chores

  • Config validation, atomic config writes, launch settings and dependency updates.