providers is a panel, not a section in officer's settings

`IntegrationsSettings/index.tsx` has carried the rule since email was extracted
on 2026-08-18: a plugin contributes panels, not settings sections. the
extraction brief said the opposite and the brief was wrong.

settings living in the platform mean this plugin is not self-contained — it
cannot be configured on an older platform, and uninstalling would leave a dead
"Atelier" section in the settings screen for someone to delete by hand.

so: a fourth panel, `atelier-providers`, and the default layout stacks it under
Collections in the right column. smallest and last because it is configured once
and then ignored — but it has to be REACHABLE, which is the whole point.

Generate's empty state pointed at "Settings → Integrations → Atelier". it now
points at the Providers panel, which is where the key actually goes.

web/Generate/Generate.tsx

@@ -15,7 +15,7 @@ export const Generate = () => {
     return (
       <Card>
         <CardContent className="pt-6 text-sm text-muted-foreground">
-          No provider is ready. Add an API key in Settings → Integrations → Atelier first.
+          No provider is ready. Add an API key in the Providers panel first.
         </CardContent>
       </Card>
     );