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__BACKENDS.md

@@ -55,7 +55,7 @@ Each response shows a model tag (bottom-right of the message bubble) with the mo
 
 Per-user configuration stored in `home/{user}/model_registry.json`.
 
-Managed at **Settings → Model Registry** (`/settings/local`). Full provider UI coming in Phase 2.
+Managed at **Settings → Models** (`/settings/models`). Full provider UI coming in Phase 2.
 
 ```json
 {