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.

lib/types/model-config.ts

@@ -60,7 +60,9 @@ export interface MultiModelConfig {
     version: 1
     providers: ProviderConfig[]
     selectedModelId?: string // Currently selected model's UUID
-    showUnvalidatedModels?: boolean // Show models that haven't been validated
+    // Older versions saved here whether untested models were listed in the
+    // model picker; they always are now
+    showUnvalidatedModels?: boolean
 }
 
 // Flattened model for dropdown display