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/dashboard/QuickActions.tsx

@@ -6,19 +6,19 @@ import { FolderPlus, Clock, Sparkles, CalendarSync } from "lucide-react";
 const actions = [
   {
     label: "Neue Akte",
-    href: "/akten?new=1",
+    href: "/cases/new",
     icon: FolderPlus,
     color: "text-blue-600 bg-blue-50 hover:bg-blue-100",
   },
   {
     label: "Frist eintragen",
-    href: "/fristen?new=1",
+    href: "/fristen",
     icon: Clock,
     color: "text-amber-600 bg-amber-50 hover:bg-amber-100",
   },
   {
     label: "AI Analyse",
-    href: "/ai",
+    href: "/ai/extract",
     icon: Sparkles,
     color: "text-violet-600 bg-violet-50 hover:bg-violet-100",
   },