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/backups-section.tsx

@@ -29,7 +29,7 @@ function formatBytes(bytes: number, fmt: ReturnType<typeof useFormatters>) {
   return `${fmt.number(bytes / 1024 / 1024, 2)} MB`;
 }
 
-export function BackupsSection({ id }: { id?: string } = {}) {
+export function BackupsSection() {
   const { t } = useTranslations();
   const fmt = useFormatters();
   const queryClient = useQueryClient();
@@ -72,10 +72,7 @@ export function BackupsSection({ id }: { id?: string } = {}) {
   const rows: BackupRow[] = data?.rows ?? [];
 
   return (
-    <div
-      id={id}
-      className="bg-card border-border scroll-mt-28 rounded-xl border p-6"
-    >
+    <div className="bg-card border-border rounded-xl border p-6">
       <div className="flex items-center justify-between gap-2">
         <div className="flex items-center gap-2">
           <Database className="text-primary h-5 w-5" />