feat: model registry Phase 2 — cloud provider UI (Anthropic + Google)

Adds cloud provider management to /settings/models:
- Google Accounts section: add/remove Gemini API keys with labels
- Add Model form: provider tabs (Local / Google / Anthropic) with
  catalog dropdowns that auto-fill label and context_k
- Provider badges on model rows (Anthropic / Google / Local)
- /settings/local now redirects to /settings/models (canonical URL)
- save_cloud_model() in model_registry for Anthropic/Google entries
- Distill role migration restored in _migrate_from_local_llm
- Test fixes: version assertions updated to V2

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/ARCH__CHANNELS.md

@@ -33,7 +33,7 @@ Single-page app served from `cortex/static/`. All chat happens via `POST /chat`
 
 **Files panel:** Browse and edit persona markdown files in-browser. Session search at the bottom.
 
-**Settings:** `/settings` — Gemini API key, Google account, connected status. `/settings/local` — local model hosts and models.
+**Settings:** `/settings` — Gemini API key, Google account, connected status. `/settings/models` — model registry (providers, hosts, models, roles).
 
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