feat(settings): reorganize the settings dialog and the model settings

Tabs Models, General, Drawing and About, each with a fixed title row.

The models tab becomes three pages: the user's providers with one status
each and the server's models; a searchable picker of all 24 providers in
four groups; and a provider page in three steps (connection, models,
test). A new provider gets its first suggested model, and a passing test
offers to use the model in the chat. Bedrock asks for one way of signing
in, with an optional session token for temporary access keys.

The composer's model picker lists every model, marking untested and
failed ones, and can open the provider picker. A chat error that offers
model settings opens the provider the request went to.

docs/en/admin-panel.md

@@ -9,7 +9,7 @@ Instead of hand-editing `.env`, you can manage server settings in a web admin pa
 
 ## What you can configure
 
-1. **Models** — add providers with their API keys and model lists, using the same UI as the in-app model settings. Saved models become server-side models available to all users, merged with any `AI_MODELS_CONFIG` / `ai-models.json` from your environment at request time (the panel does not modify those env files).
+1. **Models** — add providers with their API keys and model lists; the credential fields are the same as in the in-app model settings. Saved models become server-side models available to all users, merged with any `AI_MODELS_CONFIG` / `ai-models.json` from your environment at request time (the panel does not modify those env files).
 2. **Other sections** — access codes, generation parameters, features, observability, and quota. Saved settings are written to `data/settings.json` and apply immediately — no restart needed (a few settings such as Langfuse and DynamoDB are marked "Restart Required").
 
 ## Precedence