Legacy in-app Anthropic OAuth removed

What changed

The legacy in-app Anthropic web-OAuth flow has been removed. Anthropic providers that were configured via that OAuth flow (auth.kind === 'oauth') are re-seated to api-key mode by the v1→v2 migrator, and the renderer no longer offers the OAuth toggle for the anthropic provider.

Why this matters to the user

A user who previously signed in to Anthropic through the in-app OAuth button will find that provider in api-key mode after upgrading, with no key set — model requests will fail until a key is entered.

What the user should do

Open Settings → Providers → Anthropic and enter an Anthropic API key.

Notes for release manager

Pair with 2026-05-18-aws-bedrock-api-key-auth-renamed.md (same PR, provider-settings follow-up).