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/lib/settings-tabs.ts

@@ -0,0 +1,36 @@
+import type { ContentFieldDef } from "@/lib/content-registry";
+
+/**
+ * Reiter von Admin → Einstellungen und welche Inhaltsgruppen
+ * (lib/content-registry.ts) sie tragen. "system" hat keine Inhaltsfelder,
+ * sondern Anbindungsstatus und E-Mail-Test.
+ */
+export const SETTINGS_TABS = [
+  { id: "firma", groups: ["company", "shipping"] },
+  { id: "marke", groups: ["brand"] },
+  { id: "startseite", groups: ["home"] },
+  { id: "seiten", groups: ["pages"] },
+  { id: "social", groups: ["social"] },
+  { id: "recht", groups: ["legal"] },
+  { id: "system", groups: [] },
+] as const satisfies readonly { id: string; groups: readonly ContentFieldDef["group"][] }[];
+
+export type SettingsTabId = (typeof SETTINGS_TABS)[number]["id"];
+
+export const SETTINGS_PATH = "/admin/einstellungen";
+
+/** Reiter aus ?reiter=…; alles Unbekannte (auch Mehrfachwerte) faellt auf den ersten Reiter zurueck. */
+export function parseSettingsTab(value: string | string[] | undefined): SettingsTabId {
+  const match = SETTINGS_TABS.find((tab) => tab.id === value);
+  return match ? match.id : SETTINGS_TABS[0].id;
+}
+
+/**
+ * Ziel nach dem Speichern: derselbe Reiter. Der group-Wert kommt aus dem
+ * Formular und ist damit Nutzereingabe — nur bekannte Gruppen fuehren zu
+ * einem Reiter, alles andere zur Uebersicht.
+ */
+export function settingsPathForGroup(group: string): string {
+  const tab = SETTINGS_TABS.find((t) => (t.groups as readonly string[]).includes(group));
+  return tab ? `${SETTINGS_PATH}?reiter=${tab.id}` : SETTINGS_PATH;
+}