refactor(ui): give Settings and Admin one card rhythm and one page header

The two consoles used the same primitives and still read as two systems,
because everything without a primitive had drifted: the header-to-body
step existed in three competing techniques at once (`mt-4` on the first
body child, `space-y-4` on the card, `mb-4` on the header), the body
started either at the card edge or 28 px in, and both shells hand-rolled
the page title instead of calling `PageHeader`.

Move the decisions into the primitives so a call site cannot re-open them:

- `SettingsCard` is gap-based like `Card` (`flex flex-col gap-4`). The
  header-to-body step is the container's, not a margin somebody remembers.
  A dense card still overrides the gap on the card itself.
- The body starts at the card edge. `pl-7` is gone — off the spacing scale,
  and it mis-aligned action rows against their own forms wherever two
  neighbouring cards disagreed about it.
- The card header's description slot loses its `space-y-1`. That was the
  licence call sites used to stack explainer paragraphs into a muted
  `text-xs` slot; the slot holds one sentence.
- New `SettingsCardActions`: one action row, trailing edge, last thing in
  the card. `align="start"` spells the one exception, a card whose only
  actions are links out.
- New `StatTile` replaces four hand-rolled label/value boxes in Admin.
- `SectionHeading` moves from `insights/` to `ui/`. It is the heading above
  a group of cards, which is what Settings needs; Settings had been using a
  card header with no card, which reads as a card that failed to paint.

Both shells now render their heading through `PageHeader`, which gained
`headingAs="div"` for the second copy each shell paints (mobile above the
nav strip, desktop inside the grid) so only one is a real `<h1>`. The
page explainer is muted everywhere: the primitive said foreground, the
standards and every hand-rolled copy said muted, so pages disagreed with
each other. Medications, Vorsorge, the labs / custom-metric / illness
detail pages and the clinician view drop their hand-rolled title blocks
too, including the mid-word truncation on the medications subtitle.

Tests pin the parts that can be checked exactly: the card's gap, the
description slot's missing rhythm, the action row's two alignments, the
`<h1>`-free shells, and a sweep asserting `pl-7` stays gone.

src/components/admin/admin-shell.tsx

@@ -37,6 +37,7 @@ import {
 } from "lucide-react";
 
 import { cn } from "@/lib/utils";
+import { PageHeader } from "@/components/ui/page-header";
 import { SUB_SHELL_GRID_FLOOR } from "@/components/layout/shell-metrics";
 import { useAuth } from "@/hooks/use-auth";
 import { useTranslations } from "@/lib/i18n/context";
@@ -253,21 +254,15 @@ export function AdminShell({ active, children }: AdminShellProps) {
       ? t("admin.subtitle")
       : t(`admin.section.${activeSlug}.subtitle`);
 
+  // Same two-copy heading as the Settings shell (mobile above the chip strip,
+  // desktop inside the grid row) and the same primitive, so the two consoles
+  // cannot drift a step apart again.
   const headingBlock = (mobile = false) => (
-    <div>
-      {mobile ? (
-        <div
-          role="heading"
-          aria-level={1}
-          className="text-2xl font-bold tracking-tight"
-        >
-          {headingTitle}
-        </div>
-      ) : (
-        <h1 className="text-2xl font-bold tracking-tight">{headingTitle}</h1>
-      )}
-      <p className="text-muted-foreground text-sm">{headingSubtitle}</p>
-    </div>
+    <PageHeader
+      headingAs={mobile ? "div" : "h1"}
+      title={headingTitle}
+      description={headingSubtitle}
+    />
   );
 
   // v1.4.25 W8 — AuthShell wraps the page in `px-4 py-6 md:px-6`