feat(settings): choose the AI usage level, and set its bounds as an operator

Settings, AI gains one card with the three levels, each with what it does
and the server's estimate of tokens per day, labelled as one. Levels above
the operator's ceiling are shown disabled with the reason; a capped choice
is named and kept; a stored choice can return to the server default. The
automatic document read says when the frugal level rests it. Admin, Coach
gains the ceiling, the default and the sync gap, and the provider readout
the accounts per level. Strings in all seven languages.

The admin card's save confirms with a success toast like its neighbour
assistant-section; it is a settings write without counts, so it is pinned
in the success-affordance guard at one.

Refs #1217

src/app/admin/[section]/renderer.tsx

@@ -22,6 +22,7 @@ import { AiServerKeySection } from "@/components/admin/ai-server-key-section";
 import { ProviderHealthSection } from "@/components/admin/provider-health-section";
 import { CentralCodexSection } from "@/components/admin/central-codex-section";
 import { AssistantSection } from "@/components/admin/assistant-section";
+import { AiUsageSection } from "@/components/admin/ai-usage-section";
 import { CoachFeedbackSection } from "@/components/admin/coach-feedback-section";
 import { ApiTokenOverviewSection } from "@/components/admin/api-token-overview-section";
 import { AppLogPreviewSection } from "@/components/admin/app-log-preview-section";
@@ -102,6 +103,7 @@ export function AdminSectionRenderer({
       return (
         <SectionFrame>
           <AssistantSection />
+          <AiUsageSection />
           <AiServerKeySection />
           <CentralCodexSection />
           <ProviderHealthSection />