revert(settings): restore the flat settings navigation

Revert the nine-group consolidation and the card relocations back to the
flat sidebar: sessions, trusted devices, and login activity return to
Privacy, and account deletion returns inline under Advanced. Drop the
/settings/layout redirect and the now-unused group and sign-in cross-link
i18n keys across all six locales.

Keep the collapse-by-default sessions and activity cards, the medical-history
cross-link from "About me", and the chronic-conditions consolidation.

src/components/settings/security-section/__tests__/security-section-signin.test.tsx

@@ -1,70 +0,0 @@
-/**
- * v1.25.1 (H1) — sign-in management consolidation.
- *
- * Active sessions, trusted devices, and the login-activity feed moved out of
- * the Data & Privacy group into Account → Security so every "who can sign in as
- * me" control sits in one place. This SSR test pins that the three cards render
- * inside `<SecuritySection>`.
- */
-import { describe, it, expect, vi } from "vitest";
-import { renderToStaticMarkup } from "react-dom/server";
-import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
-
-vi.mock("@/hooks/use-auth", () => ({
-  useAuth: () => ({ isAuthenticated: true, isLoading: false }),
-}));
-
-import { I18nProvider } from "@/lib/i18n/context";
-import { queryKeys } from "@/lib/query-keys";
-import { SecuritySection } from "../index";
-
-function render() {
-  const client = new QueryClient({
-    defaultOptions: {
-      queries: {
-        retry: false,
-        staleTime: Number.POSITIVE_INFINITY,
-        refetchOnMount: false,
-      },
-    },
-  });
-  // Seed the MFA status so the second-factor cards paint instead of skeletons.
-  client.setQueryData(queryKeys.mfaStatus(), {
-    totp: { enabled: false },
-    recoveryCodesRemaining: 0,
-    webauthn: [],
-    passkeyNudgeDismissed: true,
-  });
-  client.setQueryData(queryKeys.passkeys(), []);
-  // Seed the embedded sign-in reads so the cards paint without firing network.
-  client.setQueryData(queryKeys.sessions(), { sessions: [] });
-  client.setQueryData(queryKeys.securityActivity(), { events: [] });
-  client.setQueryData(queryKeys.trustedDevices(), {
-    devices: [
-      {
-        id: "td-1",
-        label: "Firefox on macOS",
-        isCurrent: true,
-        createdAt: "2099-01-01T00:00:00.000Z",
-        lastUsedAt: "2099-01-01T00:00:00.000Z",
-        expiresAt: "2099-01-01T00:00:00.000Z",
-      },
-    ],
-  });
-  return renderToStaticMarkup(
-    <I18nProvider initialLocale="en">
-      <QueryClientProvider client={client}>
-        <SecuritySection />
-      </QueryClientProvider>
-    </I18nProvider>,
-  );
-}
-
-describe("<SecuritySection> — sign-in management home (H1)", () => {
-  it("renders the active-session, trusted-device, and security-activity cards", () => {
-    const html = render();
-    expect(html).toContain('data-slot="settings-security-sessions-card"');
-    expect(html).toContain('data-slot="settings-trusted-devices-card"');
-    expect(html).toContain('data-slot="settings-security-activity-card"');
-  });
-});