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/lib/insights/bucket-series.ts

@@ -16,10 +16,8 @@
  * does not produce 360 zero-rows of noise.
  */
 
-const BERLIN_TZ = "Europe/Berlin";
-
 const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: BERLIN_TZ,
+  timeZone: "Europe/Berlin",
   year: "numeric",
   month: "2-digit",
   day: "2-digit",

src/lib/insights/mood-status.ts

@@ -17,8 +17,6 @@ import {
 } from "@/lib/insights/bucket-series";
 import { annotate } from "@/lib/logging/context";
 
-const MS_PER_DAY = 24 * 60 * 60 * 1000;
-
 const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
   timeZone: "Europe/Berlin",
   year: "numeric",
@@ -285,7 +283,7 @@ export async function generateMoodStatusForUser(
   // ~90-day window so the model still gets a recency-weighted view of
   // tag patterns. The mood DB query already pulls 3 years (no where
   // filter); we slice by date here, not by record count.
-  const tagWindowCutoff = new Date(now.getTime() - 90 * MS_PER_DAY);
+  const tagWindowCutoff = new Date(now.getTime() - 90 * 24 * 60 * 60 * 1000);
   const recentEntries = entries.filter(
     (entry) => entry.moodLoggedAt.getTime() >= tagWindowCutoff.getTime(),
   );