feat(settings): standardize settings and admin section framing

Introduce a shared SettingsSectionFrame (the settings twin of the admin
renderer's SectionFrame) that emits the canonical visible heading +
subheading, and route all 19 settings sections through it. Every settings
page now shows a real h1 + subtitle in the reference Übersicht style
instead of an invisible sr-only title; the section bodies drop their own
headings and off-style intro paragraphs (the explanatory copy folds into
the frame subtitle).

- Route the admin About page through SectionFrame so it carries the same
  visible heading as every other admin section.
- Rename the AI settings section from KI-Auswertung to KI-Anbieter across
  six locales: the page configures the provider and key, not an analysis.
- Add a heading-block spacer to the settings and admin sidebars so the
  first menu item lines up with the top of the first card.
- Remove the always-on core-domains card from Module: a tracking area that
  can't be turned off doesn't need listing.
- Move the proactive-coach control out of Benachrichtigungen into Coach.
- Move admin module availability out of the Coach section into its own
  page: it gates every module, not just the coach.
- Give the Layout Insights editor a proper Insights heading.

Adds settings.sections.<slug>.subtitle and the new admin keys across all
six locales; removes the now-unused module core-domain strings.

src/components/settings/api-section.tsx

@@ -37,19 +37,13 @@ interface ApiTokenInfo {
 }
 
 export function ApiSection() {
-  const { t } = useTranslations();
-
+  // v1.18.6 (W9) — the visible heading + subtitle now come from the shared
+  // `<SettingsSectionFrame>` in the route; this body is the API cards.
   return (
-    <section aria-labelledby="settings-section-api-title" className="space-y-6">
-      <header>
-        <h1 id="settings-section-api-title" className="sr-only">
-          {t("settings.sections.api.title")}
-        </h1>
-      </header>
-
+    <div className="space-y-6">
       <ApiEndpointsCard />
       <ApiTokensCard />
-    </section>
+    </div>
   );
 }