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/components/layout/header.tsx

@@ -1,6 +1,7 @@
 'use client';
 
 import { useEffect, useRef, useState } from 'react';
+import Link from 'next/link';
 import { useTranslations } from 'next-intl';
 import { useSidebarStore } from '@/lib/stores/sidebar-store';
 import { useAuthStore } from '@/lib/stores/auth-store';
@@ -128,6 +129,31 @@ export function Header() {
                 )}
               </div>
 
+              {/* Settings link */}
+              <div className="px-2 py-1">
+                <Link
+                  href="/settings"
+                  onClick={() => setDropdownOpen(false)}
+                  className="flex w-full items-center gap-2 rounded-md px-2 py-1.5 text-sm text-foreground hover:bg-muted transition-colors"
+                >
+                  <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"
+                  >
+                    <circle cx="12" cy="12" r="3" />
+                    <path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1 0 2.83 2 2 0 0 1-2.83 0l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-2 2 2 2 0 0 1-2-2v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83 0 2 2 0 0 1 0-2.83l.06-.06A1.65 1.65 0 0 0 4.68 15a1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1-2-2 2 2 0 0 1 2-2h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 0-2.83 2 2 0 0 1 2.83 0l.06.06A1.65 1.65 0 0 0 9 4.68a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 2-2 2 2 0 0 1 2 2v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 0 2 2 0 0 1 0 2.83l-.06.06A1.65 1.65 0 0 0 19.4 9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 2 2 2 2 0 0 1-2 2h-.09a1.65 1.65 0 0 0-1.51 1z" />
+                  </svg>
+                  {tHeader('settings')}
+                </Link>
+              </div>
+
               {/* Logout button */}
               <div className="px-2 py-1">
                 <button

apps/web/src/components/settings/settings-sidebar.tsx

@@ -0,0 +1,64 @@
+'use client';
+
+import Link from 'next/link';
+import { usePathname } from 'next/navigation';
+import { useTranslations } from 'next-intl';
+
+/**
+ * Settings sub-sidebar (D-19, D-20).
+ * Phase 05 categories: Dashboard > Widgets, Calendar.
+ * Active item uses sidebar-accent pattern with aria-current="page".
+ */
+export function SettingsSidebar() {
+  const t = useTranslations('settings');
+  const pathname = usePathname();
+
+  const items = [
+    {
+      label: t('categoryWidgets'),
+      href: '/settings/dashboard',
+    },
+    {
+      label: t('categoryCalendar'),
+      href: '/settings/dashboard/calendar',
+    },
+  ];
+
+  const isActive = (href: string) => {
+    if (href === '/settings/dashboard') {
+      // Active when exactly on /settings/dashboard (not on sub-routes like /calendar)
+      return pathname === '/settings/dashboard';
+    }
+    return pathname.startsWith(href);
+  };
+
+  return (
+    <aside
+      className="flex w-[220px] shrink-0 flex-col border-r border-sidebar-border bg-sidebar"
+      role="navigation"
+      aria-label={t('navLabel')}
+    >
+      <div className="p-4">
+        <h2 className="text-xs font-semibold uppercase tracking-wider text-muted-foreground">
+          {t('categoryDashboard')}
+        </h2>
+      </div>
+      <nav className="flex flex-col gap-1 px-3">
+        {items.map((item) => (
+          <Link
+            key={item.href}
+            href={item.href}
+            className={`flex items-center rounded-md px-2 py-1.5 text-sm transition-colors ${
+              isActive(item.href)
+                ? 'bg-sidebar-accent text-sidebar-accent-foreground font-medium'
+                : 'text-sidebar-foreground hover:bg-muted'
+            }`}
+            aria-current={isActive(item.href) ? 'page' : undefined}
+          >
+            {item.label}
+          </Link>
+        ))}
+      </nav>
+    </aside>
+  );
+}