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/page.tsx

@@ -20,12 +20,12 @@ export default function FristenPage() {
   });
 
   return (
-    <div className="space-y-4">
-      <div className="flex items-center justify-between">
+    <div className="animate-fade-in space-y-4">
+      <div className="flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
         <div>
           <h1 className="text-lg font-semibold text-neutral-900">Fristen</h1>
           <p className="mt-0.5 text-sm text-neutral-500">
-            Alle Fristen im Uberblick
+            Alle Fristen im Überblick
           </p>
         </div>
         <div className="flex items-center gap-2">