feat(admin): settings page with tabs, system checks and email test

"Inhalte" becomes Admin -> Einstellungen, grouped into tabs so each
area stands on its own: Firma & Versand, Marke, Startseite, Seiten,
Social Media, Rechtstexte and System & Tests. Tabs are plain links
(?reiter=...), bookmarkable and usable without JavaScript; saving
returns to the same tab.

System & Tests brings everything that checks the running shop into one
place:
- integration overview (Stripe, Stripe webhook, Microsoft 365 mail,
  Microsoft sign-in, Turnstile, Google reviews, DHL, ApiCore): only
  whether each is set up and the names of missing variables, never a
  value; warns when Stripe runs on test keys or mixes test/live
- the email test, now with the IPv4 network checks for
  smtp.office365.com:587 and login.microsoftonline.com:443

/admin/inhalte and /admin/e-mail-test redirect to the new page; the
separate "E-Mail-Test" menu entry is gone. The tab parameter and the
saved group are validated against a fixed list before they are used in
a redirect. Docs point to the new location and cover the ENETUNREACH
(IPv6) case.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

src/components/admin-shell.tsx

@@ -57,9 +57,8 @@ function getAdminNavGroups(t: Dictionary): { label: string | null; items: NavIte
     {
       label: t.adminShell.groupSite,
       items: [
-        { href: "/admin/inhalte", label: t.adminShell.navContent },
+        { href: "/admin/einstellungen", label: t.adminShell.navContent },
         { href: "/admin/audit-log", label: t.adminShell.navAuditLog },
-        { href: "/admin/e-mail-test", label: t.adminShell.navEmailTest },
       ],
     },
   ];