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/app/globals.css

@@ -10,6 +10,19 @@ body {
   -moz-osx-font-smoothing: grayscale;
 }
 
+/* Focus-visible ring for accessibility */
+*:focus-visible {
+  outline: 2px solid #404040;
+  outline-offset: 2px;
+  border-radius: 4px;
+}
+
+input:focus-visible,
+select:focus-visible,
+textarea:focus-visible {
+  outline: none;
+}
+
 @keyframes count-up {
   0% {
     transform: translateY(8px);
@@ -24,3 +37,31 @@ body {
 .animate-count-up {
   animation: count-up 0.3s ease-out;
 }
+
+@keyframes fade-in {
+  0% {
+    opacity: 0;
+    transform: translateY(4px);
+  }
+  100% {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+.animate-fade-in {
+  animation: fade-in 0.2s ease-out;
+}
+
+@keyframes slide-in-left {
+  0% {
+    transform: translateX(-100%);
+  }
+  100% {
+    transform: translateX(0);
+  }
+}
+
+.animate-slide-in-left {
+  animation: slide-in-left 0.2s ease-out;
+}