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/(app)/fristen/rechner/page.tsx

@@ -6,18 +6,20 @@ import Link from "next/link";
 
 export default function FristenrechnerPage() {
   return (
-    <div className="space-y-4">
+    <div className="animate-fade-in space-y-4">
       <div>
         <Link
           href="/fristen"
-          className="mb-2 inline-flex items-center gap-1 text-sm text-neutral-500 hover:text-neutral-700"
+          className="mb-2 inline-flex items-center gap-1 text-sm text-neutral-500 transition-colors hover:text-neutral-700"
         >
           <ArrowLeft className="h-3.5 w-3.5" />
-          Zuruck zu Fristen
+          Zurück zu Fristen
         </Link>
-        <h1 className="text-lg font-semibold text-neutral-900">Fristenrechner</h1>
+        <h1 className="text-lg font-semibold text-neutral-900">
+          Fristenrechner
+        </h1>
         <p className="mt-0.5 text-sm text-neutral-500">
-          Berechnen Sie Fristen basierend auf Verfahrensart und Auslosedatum
+          Berechnen Sie Fristen basierend auf Verfahrensart und Auslösedatum
         </p>
       </div>
       <DeadlineCalculator />