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/section-slugs.ts

@@ -28,25 +28,3 @@ export type AdminSectionSlug = (typeof ADMIN_SECTION_SLUGS)[number];
 export function isAdminSectionSlug(value: string): value is AdminSectionSlug {
   return (ADMIN_SECTION_SLUGS as readonly string[]).includes(value);
 }
-
-/**
- * Legacy hash-anchor → new slug mapping. Used by `<StatusCardGrid>` to
- * rewrite hrefs and by `proxy.ts` to handle any server-side legacy URL
- * forms (path-style anchors are redirectable; pure `#fragments` never
- * reach the server, so those are fixed at the call-sites instead).
- */
-export const LEGACY_ANCHOR_TO_SLUG: Record<string, AdminSectionSlug> = {
-  "section-system-status": "system-status",
-  "section-admin-general": "general",
-  "section-admin-services": "services",
-  "section-admin-umami": "integrations",
-  "section-admin-glitchtip": "integrations",
-  "section-admin-webpush": "integrations",
-  "section-admin-bugreport": "integrations",
-  "section-admin-feedback": "feedback",
-  "section-admin-reminders": "reminders",
-  "section-user-management": "users",
-  "section-api-tokens": "api-tokens",
-  "section-login-overview": "login-overview",
-  "section-danger-zone": "danger-zone",
-};