feat(05-01): settings shell, header link, and i18n keys for dashboard phase

- Create settings/layout.tsx with SettingsSidebar and back-to-dashboard link
- Create settings/page.tsx with redirect to /settings/dashboard
- Create settings-sidebar.tsx with Widgets and Calendar nav items, aria-current
- Add Settings link in header user dropdown (gear icon, before logout)
- Add settings namespace (DE+EN) with all category and action keys
- Add widgets namespace (DE+EN) with all widget names, descriptions, error states
- Add header.settings key ("Einstellungen"/"Settings")

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/app/(portal)/settings/layout.tsx

@@ -0,0 +1,47 @@
+'use client';
+
+import Link from 'next/link';
+import { useTranslations } from 'next-intl';
+import { SettingsSidebar } from '@/components/settings/settings-sidebar';
+
+/**
+ * Settings nested layout (D-19, D-20).
+ * Renders within the portal AppShell, adding its own sub-sidebar on the left.
+ * Back-to-dashboard link at top of content area per UI-SPEC.
+ */
+export default function SettingsLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  const t = useTranslations('settings');
+
+  return (
+    <div className="flex h-full">
+      <SettingsSidebar />
+      <div className="flex-1 overflow-y-auto p-6">
+        {/* Back to Dashboard link */}
+        <Link
+          href="/"
+          className="mb-4 inline-flex items-center gap-1 text-sm text-muted-foreground transition-colors hover:text-foreground"
+        >
+          <svg
+            xmlns="http://www.w3.org/2000/svg"
+            width="16"
+            height="16"
+            viewBox="0 0 24 24"
+            fill="none"
+            stroke="currentColor"
+            strokeWidth="2"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+          >
+            <polyline points="15 18 9 12 15 6" />
+          </svg>
+          {t('backToDashboard')}
+        </Link>
+        {children}
+      </div>
+    </div>
+  );
+}

apps/web/src/app/(portal)/settings/page.tsx

@@ -0,0 +1,8 @@
+import { redirect } from 'next/navigation';
+
+/**
+ * Settings index page — redirects to /settings/dashboard (the default category).
+ */
+export default function SettingsPage() {
+  redirect('/settings/dashboard');
+}