feat(providers): add Claude Pro/Max subscription OAuth (PKCE-localhost)

27th registry variant, 2nd subscription-OAuth provider. Log in with a Claude
Pro/Max subscription via PKCE authorization-code + S256 (loopback callback on
127.0.0.1:53692, with a manual code-paste fallback), then use it against
api.anthropic.com — reusing the existing Anthropic Messages wire + Anthropic
listing + the multi-provider OAuth foundation (dbce6bf). Sourced from Pi
(earendil-works/pi auth/oauth/anthropic.ts): client 9d1c250a..., authorize
claude.ai/oauth/authorize, token platform.claude.com/v1/oauth/token, scope
'…user:inference user:sessions:claude_code…'.

New machinery (foundation handles token routing — claude-pro-max is a
uses_oauth platform so its bearer/refresh/api_key already route to its own
pooled manager, never Kimi):
- OAuthConfig gains flow{DeviceCode|PkceLocalhost} + token_host + token_body
  {Form|JSON}; xai/kimi rows unchanged (DeviceCode/Form).
- auth/oauth_pkce.rs: PKCE S256 wire — loopback listener with STRICT state
  validation (CSRF, fail-closed), manual-paste fallback, JSON code→token
  exchange + rotating-refresh. Never logs code/verifier/tokens.
- Messages OAuth adaptation gated on SamplerConfig.anthropic_oauth (true only
  for a claude-pro-max managed key): Authorization: Bearer + anthropic-beta
  oauth + user-agent claude-cli + x-app cli, and the required 'You are Claude
  Code' system prefix. API-key anthropic/minimax Messages requests are
  BYTE-IDENTICAL (regression-guarded).
- Live /models under the OAuth Bearer + oauth-beta headers (Anthropic listing,
  enriched from models.dev anthropic); persistent 401 → 0 models + WARN, NO
  hardcoded fallback list (honest failure).

Adversarial review: no blocking findings (secret handling, CSRF/state, the
anthropic_oauth gate, token routing, non-regression all CONFIRMED). Full gate
green. Registry at 27; picker updated. Residual (unverifiable without a real
Claude Pro/Max account): whether GET /v1/models accepts the OAuth bearer, and
the real endpoint's acceptance of the OAuth Messages request.

crates/codegen/kigi-tui/src/views/welcome/mod.rs

@@ -2106,6 +2106,14 @@ mod tests {
         // lists ~20 platforms); this asserts content coverage, not fit.
         let text = render_done_text_h(&params, 72);
         assert!(text.contains("Kimi Code (OAuth)"), "{text}");
+        assert!(
+            text.contains("xAI Grok (subscription) (OAuth)"),
+            "the xai-grok interactive OAuth login row must render: {text}"
+        );
+        assert!(
+            text.contains("Claude Pro/Max (subscription) (OAuth)"),
+            "the claude-pro-max interactive OAuth login row must render: {text}"
+        );
         assert!(
             text.contains("Moonshot Open Platform (API key \u{b7} moonshot.cn)"),
             "{text}"