refactor(admin): the console speaks the Settings vocabulary without an accent

Twenty-six of twenty-eight admin files already used the Settings card and
header. What made the console read as its own system was everything on top of
that: three hand-rolled cards, a header slot that meant five different things,
and a heading the frame took but never painted.

- `SectionFrame` drops `title` and `subtitle`. Seventeen call sites passed
  both into a component that has rendered neither since v1.18.6.1 — the shell
  reads the same keys itself. A prop contract nothing honours is a promise the
  next reader believes.
- The invite card, the four host-metric headers, and the `/admin` welcome
  card stop hand-rolling `bg-card border rounded-xl` shells with their own
  padding steps and their own heading weights. `text-sm` as a card title on
  the first card of a page, directly above a correct one, was the first thing
  the eye landed on.
- The header status slot carries status. The backups run, the invite create,
  the danger-zone wipe, and the user filters come out of it; the app-log
  refresh stays but reads as the secondary it is.
- Loading states paint their card and header. Encryption returned a bare
  spinner row and then a card, so the section jumped when the status landed —
  the fix `coach-feedback-section.tsx` already carried, applied where it was
  missing. Same for the notification-status card.
- One label/value tile. `StatusItem`, the encryption section's private `Stat`,
  the user-management inline panels and the backups upload box were four
  fills and three paddings for one role.
- The overview stopped printing the running version, build SHA and build time
  twice, thirty pixels apart. The version tile owns build identity — it is the
  one that checks for a newer release; the snapshot owns what is running.
- `ai-server-key` had three paragraphs stacked in a muted `text-xs` slot;
  one sentence stays there and the two that carry information move into the
  body as foreground. The central-codex consent copy moves the same way, and
  its disconnect is `variant="destructive"` — which `ConfirmButton` refused to
  accept, so every destructive confirm in the tree had been hand-tinting an
  outline instead.
- Leftovers: the `-mt-2` correcting the card's own rhythm, two alpha hovers on
  text, `text-[10px]` badges, the redundant `space-y-6` wrapper, the empty
  state repeating the header's CTA, and the `mt-6` a card added inside a
  frame that already spaces its children.

src/components/admin/general-settings-section.tsx

@@ -103,7 +103,7 @@ export function GeneralSettingsSection() {
         {(settings?.registrationEnabled ?? true) && (
           <p
             data-slot="registration-open-hint"
-            className="text-muted-foreground -mt-2 text-xs"
+            className="text-muted-foreground text-xs"
           >
             {t("admin.registrationOpenHint")}
           </p>