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/cases/CaseTimeline.tsx

@@ -3,6 +3,7 @@
 import type { CaseEvent } from "@/lib/types";
 import { format } from "date-fns";
 import { de } from "date-fns/locale";
+import { Activity } from "lucide-react";
 
 const EVENT_ICONS: Record<string, string> = {
   case_created: "bg-emerald-500",
@@ -20,9 +21,14 @@ interface CaseTimelineProps {
 export function CaseTimeline({ events }: CaseTimelineProps) {
   if (events.length === 0) {
     return (
-      <p className="py-8 text-center text-sm text-neutral-400">
-        Keine Ereignisse vorhanden.
-      </p>
+      <div className="flex flex-col items-center py-8 text-center">
+        <div className="rounded-xl bg-neutral-100 p-3">
+          <Activity className="h-5 w-5 text-neutral-400" />
+        </div>
+        <p className="mt-2 text-sm text-neutral-500">
+          Keine Ereignisse vorhanden.
+        </p>
+      </div>
     );
   }