feat: UI polish — responsive, loading/empty/error states, German fixes (Phase 3Q)

- Responsive sidebar: collapses on mobile with hamburger menu, slide-in animation
- Skeleton loaders: dashboard cards, case table, case detail page
- Empty states: friendly messages with icons for cases, deadlines, parties, documents
- Error states: retry button on dashboard, proper error message on case not found
- Form validation: inline error messages on case creation form
- German language: fix all missing umlauts (Zurück, wählen, Anhängig, Verfügung, etc.)
- Status labels: display German translations instead of raw status values
- Transitions: fade-in animations on page load, hover/transition-colors on all interactive elements
- Focus states: focus-visible ring for keyboard accessibility
- Mobile layout: stacking for filters, forms, tabs; horizontal scroll for tables
- Extraction results: card layout on mobile, table on desktop
- Missing types: add DashboardData, DeadlineSummary, CaseSummary, ExtractedDeadline etc.
- Fix QuickActions links to use correct routes (/cases/new, /ai/extract)
- Consistent input focus styles across all forms

frontend/src/components/layout/TenantSwitcher.tsx

@@ -12,17 +12,20 @@ export function TenantSwitcher() {
   const ref = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
-    api.get<TenantWithRole[]>("/tenants").then((data) => {
-      setTenants(data);
-      const savedId = localStorage.getItem("kanzlai_tenant_id");
-      const match = data.find((t) => t.id === savedId) || data[0];
-      if (match) {
-        setCurrent(match);
-        localStorage.setItem("kanzlai_tenant_id", match.id);
-      }
-    }).catch(() => {
-      // Not authenticated or no tenants
-    });
+    api
+      .get<TenantWithRole[]>("/tenants")
+      .then((data) => {
+        setTenants(data);
+        const savedId = localStorage.getItem("kanzlai_tenant_id");
+        const match = data.find((t) => t.id === savedId) || data[0];
+        if (match) {
+          setCurrent(match);
+          localStorage.setItem("kanzlai_tenant_id", match.id);
+        }
+      })
+      .catch(() => {
+        // Not authenticated or no tenants
+      });
   }, []);
 
   useEffect(() => {
@@ -48,14 +51,16 @@ export function TenantSwitcher() {
     <div ref={ref} className="relative">
       <button
         onClick={() => setOpen(!open)}
-        className="flex items-center gap-1.5 rounded-md border border-neutral-200 bg-white px-2.5 py-1.5 text-sm text-neutral-700 hover:bg-neutral-50"
+        className="flex items-center gap-1.5 rounded-md border border-neutral-200 bg-white px-2.5 py-1.5 text-sm text-neutral-700 transition-colors hover:bg-neutral-50"
       >
-        <span className="max-w-[160px] truncate">{current.name}</span>
+        <span className="max-w-[120px] truncate sm:max-w-[160px]">
+          {current.name}
+        </span>
         <ChevronsUpDown className="h-3.5 w-3.5 text-neutral-400" />
       </button>
 
       {open && tenants.length > 1 && (
-        <div className="absolute right-0 top-full z-50 mt-1 w-56 rounded-md border border-neutral-200 bg-white py-1 shadow-lg">
+        <div className="animate-fade-in absolute right-0 top-full z-50 mt-1 w-56 rounded-md border border-neutral-200 bg-white py-1 shadow-lg">
           {tenants.map((tenant) => (
             <button
               key={tenant.id}