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/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');
+}