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/admin-shell.tsx

@@ -33,17 +33,7 @@ import {
 
 import { cn } from "@/lib/utils";
 import { useTranslations } from "@/lib/i18n/context";
-import {
-  ADMIN_SECTION_SLUGS,
-  isAdminSectionSlug,
-  type AdminSectionSlug,
-} from "./section-slugs";
-
-// Re-export so callers don't have to chase two modules. New server-side
-// callers (e.g. `generateStaticParams()`) should still import from
-// `./section-slugs` directly — values exported through this client
-// boundary become unusable proxies on the server.
-export { ADMIN_SECTION_SLUGS, isAdminSectionSlug, type AdminSectionSlug };
+import { isAdminSectionSlug, type AdminSectionSlug } from "./section-slugs";
 
 interface AdminSection {
   slug: AdminSectionSlug;