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/ui/EmptyState.tsx

@@ -0,0 +1,28 @@
+import type { LucideIcon } from "lucide-react";
+
+interface EmptyStateProps {
+  icon: LucideIcon;
+  title: string;
+  description?: string;
+  action?: React.ReactNode;
+}
+
+export function EmptyState({
+  icon: Icon,
+  title,
+  description,
+  action,
+}: EmptyStateProps) {
+  return (
+    <div className="flex flex-col items-center justify-center rounded-xl border border-dashed border-neutral-300 bg-white px-6 py-12 text-center">
+      <div className="rounded-xl bg-neutral-100 p-3">
+        <Icon className="h-6 w-6 text-neutral-400" />
+      </div>
+      <h3 className="mt-3 text-sm font-medium text-neutral-900">{title}</h3>
+      {description && (
+        <p className="mt-1 max-w-sm text-sm text-neutral-500">{description}</p>
+      )}
+      {action && <div className="mt-4">{action}</div>}
+    </div>
+  );
+}