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/i18n/dictionaries/en.ts

@@ -1243,6 +1243,11 @@ export const en: Dictionary = {
     detailLabel: "Mail server message",
     configLabel: "Settings used",
     authModeOAuth: "Microsoft 365, OAuth 2.0 via the SSO app",
+    networkHeading: "Network (IPv4)",
+    networkSmtp: "smtp.office365.com, port 587",
+    networkEntra: "login.microsoftonline.com, port 443",
+    networkOk: "reachable",
+    networkFailed: "not reachable",
     missingLabel: "Missing settings",
   },
   adminWithdrawalsPage: {
@@ -1416,8 +1421,37 @@ export const en: Dictionary = {
     chartTooltip: (day, value) => `${day}: ${value}`,
   },
   adminContentPage: {
+    tabsLabel: "Settings sections",
+    tabCompany: "Company & shipping",
+    tabBrand: "Brand",
+    tabHome: "Home page",
+    tabPages: "Pages",
+    tabSocial: "Social media",
+    tabLegal: "Legal texts",
+    tabSystem: "System & tests",
+    systemIntro: "Checks whether the shop can work with all connected services. Values from the Kubernetes secret are never shown here, only whether they are set.",
+    emailTestHeading: "Email delivery",
+    integrationsHeading: "Integrations",
+    integrationsIntro: "Which services are set up. If something is missing, the names of the missing values are listed — to be added to the Kubernetes secret or config map (see docs/EINRICHTUNG.md).",
+    statusConfigured: "Set up",
+    statusNotConfigured: "Not set up",
+    statusMissing: (names) => `Missing: ${names}`,
+    statusRequired: "Required",
+    statusOptional: "Optional",
+    warningStripeTest: "Test keys active — orders look successful, but no money is collected.",
+    warningStripeMixed: "Secret and publishable key come from different modes (test/live) — checkout will not work like this.",
+    integrationNames: {
+      stripe: "Stripe (payments)",
+      stripeWebhook: "Stripe webhook (payment confirmation)",
+      mail: "Email delivery (Microsoft 365)",
+      sso: "Sign in with Microsoft",
+      turnstile: "Cloudflare Turnstile (CAPTCHA)",
+      googleReviews: "Google reviews",
+      dhl: "DHL (shipping labels)",
+      apicore: "ApiCore sync",
+    },
     backToDashboard: "Back to dashboard",
-    title: "Content",
+    title: "Settings",
     intro:
       "These texts appear on the website. An empty field does not mean \"blank on the page\": the original text keeps showing. Only fill in what you want to change.",
     sectionImages: "Images",
@@ -1484,8 +1518,7 @@ export const en: Dictionary = {
     navVouchers: "Vouchers",
     navReviews: "Reviews",
     navAuditLog: "Audit log",
-    navEmailTest: "Email test",
-    navContent: "Content",
+    navContent: "Settings",
     navAriaLabel: "Admin navigation",
     mobileNavAriaLabel: "Mobile admin navigation",
     openMenu: "Open menu",