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/Skeleton.tsx

@@ -0,0 +1,43 @@
+export function Skeleton({ className = "" }: { className?: string }) {
+  return (
+    <div
+      className={`animate-pulse rounded-md bg-neutral-200/60 ${className}`}
+    />
+  );
+}
+
+export function SkeletonCard({ className = "" }: { className?: string }) {
+  return (
+    <div
+      className={`rounded-xl border border-neutral-200 bg-white p-5 ${className}`}
+    >
+      <Skeleton className="h-4 w-24" />
+      <div className="mt-4 space-y-3">
+        <Skeleton className="h-3 w-full" />
+        <Skeleton className="h-3 w-3/4" />
+        <Skeleton className="h-3 w-1/2" />
+      </div>
+    </div>
+  );
+}
+
+export function SkeletonTable({ rows = 5 }: { rows?: number }) {
+  return (
+    <div className="overflow-hidden rounded-md border border-neutral-200 bg-white">
+      <div className="border-b border-neutral-100 px-4 py-3">
+        <Skeleton className="h-3 w-full" />
+      </div>
+      {Array.from({ length: rows }).map((_, i) => (
+        <div
+          key={i}
+          className="flex items-center gap-4 border-b border-neutral-100 px-4 py-3 last:border-b-0"
+        >
+          <Skeleton className="h-3 w-20" />
+          <Skeleton className="h-3 flex-1" />
+          <Skeleton className="h-3 w-16" />
+          <Skeleton className="h-3 w-12" />
+        </div>
+      ))}
+    </div>
+  );
+}