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)/ai/extract/page.tsx

@@ -90,14 +90,14 @@ export default function AIExtractPage() {
 
       await Promise.all(promises);
       toast.success(
-        `${deadlines.length} Frist(en) erfolgreich uebernommen.`,
+        `${deadlines.length} Frist(en) erfolgreich übernommen.`,
       );
-      router.push(`/akten/${selectedCaseId}`);
+      router.push(`/cases/${selectedCaseId}`);
     } catch (err: unknown) {
       const message =
         err && typeof err === "object" && "error" in err
           ? (err as { error: string }).error
-          : "Uebernahme fehlgeschlagen";
+          : "Übernahme fehlgeschlagen";
       toast.error(message);
     } finally {
       setIsAdopting(false);
@@ -105,7 +105,7 @@ export default function AIExtractPage() {
   }
 
   return (
-    <div className="mx-auto max-w-4xl">
+    <div className="animate-fade-in mx-auto max-w-4xl">
       <div className="mb-6 flex items-center gap-3">
         <Brain className="h-5 w-5 text-neutral-500" />
         <div>
@@ -118,7 +118,7 @@ export default function AIExtractPage() {
         </div>
       </div>
 
-      <div className="rounded-lg border border-neutral-200 bg-white p-6">
+      <div className="rounded-lg border border-neutral-200 bg-white p-4 sm:p-6">
         <ExtractionForm
           cases={cases}
           selectedCaseId={selectedCaseId}
@@ -129,7 +129,7 @@ export default function AIExtractPage() {
       </div>
 
       {results !== null && (
-        <div className="mt-6 rounded-lg border border-neutral-200 bg-white p-6">
+        <div className="animate-fade-in mt-6 rounded-lg border border-neutral-200 bg-white p-4 sm:p-6">
           <ExtractionResults
             deadlines={results}
             onAdopt={handleAdopt}