refactor(v1.5): apply simplify-review safe-to-apply suggestions

Drops dead `id` props and `scroll-mt-28` classes from admin section
components (load-bearing in v1.4 monolithic admin, dead since the v1.5
phase-4b dynamic-route split). Also removes the unused
`LEGACY_ANCHOR_TO_SLUG` map, the `admin-shell` re-export shim, the
`BERLIN_TZ` and duplicate `MS_PER_DAY` constants, the verbose
`AdminSectionPage` docstring, the `useEffect` paraphrase comment in
`ai-section`, and rewrites the `_exhaustive: never` exhaustiveness
guard to the modern `slug satisfies never` form. `UserManagementSection`
now reads `currentUserId` via `useAuth()` instead of accepting it as a
prop.

Findings 1, 2, 3, 4, 6, 7, 8, 11, 15, 16, 17 from
.planning/phase-6-simplify-findings.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/danger-zone-section.tsx

@@ -17,7 +17,7 @@ import {
 import { Button } from "@/components/ui/button";
 import { useTranslations } from "@/lib/i18n/context";
 
-export function DangerZoneSection({ id }: { id?: string } = {}) {
+export function DangerZoneSection() {
   const { t } = useTranslations();
   const queryClient = useQueryClient();
   const [wipeMsg, setWipeMsg] = useState<string | null>(null);
@@ -65,10 +65,7 @@ export function DangerZoneSection({ id }: { id?: string } = {}) {
   });
 
   return (
-    <div
-      id={id}
-      className="bg-destructive/5 border-destructive/30 scroll-mt-28 rounded-xl border p-6"
-    >
+    <div className="bg-destructive/5 border-destructive/30 rounded-xl border p-6">
       <div className="flex items-center gap-2">
         <AlertTriangle className="text-destructive h-5 w-5" />
         <h2 className="text-destructive text-lg font-semibold">