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/settings/ai-section.tsx

@@ -214,14 +214,7 @@ function InsightsSettingsCard({
   // 15-minute device-code window expires. Polling stops as soon as
   // `deviceCode` is cleared (success / cancel / unmount).
   useEffect(() => {
-    if (!deviceCode) {
-      // No active device code → make sure the spinner is off. setState
-      // inside an effect is intentional here; the alternative (deriving
-      // `devicePolling` from `deviceCode`) would force every error
-      // path to also reset `deviceCode`, which we don't want — the
-      // start handler resets `devicePolling` itself when fetch fails.
-      return;
-    }
+    if (!deviceCode) return;
     let cancelled = false;
     const intervalMs = Math.max(deviceCode.intervalSeconds, 3) * 1000;