[Feature]: Custom-endpoint (Anthropic-compatible) accounts in AI Provider Accounts

**Problem:** I run Claude Code against several Anthropic-compatible endpoints (MiniMax, GLM, Grok, DeepSeek) via `ANTHROPIC_BASE_URL` + `ANTHROPIC_AUTH_TOKEN` + `ANTHROPIC_MODEL` set in wrapper quick commands. They work, but they can't live in **Settings → AI Provider Accounts** — that panel only accepts `subscription-oauth` Anthropic logins. So they don't show up as accounts, can't be switched between mid-session, and get no usage tracking.

**Ask:** Support an AI Provider Account of type "custom endpoint" with fields: base URL, auth token (or key reference), default model, small/fast model. Treat it like a managed account for switching + usage tracking, minus the OAuth login flow (custom endpoints self-auth via the token).

**Why likely low-lift:** the desktop bundle already references `baseUrl` / `customEndpoint` / provider-account internals — this looks like exposing existing plumbing rather than new architecture.

**Workaround today:** per-endpoint wrapper `.cmd` quick commands. Works, but no in-panel switching and no usage tracking.