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/layout/Sidebar.tsx

@@ -9,11 +9,14 @@ import {
   Calendar,
   Brain,
   Settings,
+  Menu,
+  X,
 } from "lucide-react";
+import { useState, useEffect } from "react";
 
 const navigation = [
   { name: "Dashboard", href: "/dashboard", icon: LayoutDashboard },
-  { name: "Akten", href: "/akten", icon: FolderOpen },
+  { name: "Akten", href: "/cases", icon: FolderOpen },
   { name: "Fristen", href: "/fristen", icon: Clock },
   { name: "Termine", href: "/termine", icon: Calendar },
   { name: "AI Analyse", href: "/ai/extract", icon: Brain },
@@ -22,20 +25,43 @@ const navigation = [
 
 export function Sidebar() {
   const pathname = usePathname();
+  const [mobileOpen, setMobileOpen] = useState(false);
 
-  return (
-    <aside className="flex h-full w-56 flex-col border-r border-neutral-200 bg-white">
-      <div className="flex h-14 items-center border-b border-neutral-200 px-4">
+  // Close on route change
+  useEffect(() => {
+    setMobileOpen(false);
+  }, [pathname]);
+
+  // Close on escape
+  useEffect(() => {
+    function onKeyDown(e: KeyboardEvent) {
+      if (e.key === "Escape") setMobileOpen(false);
+    }
+    document.addEventListener("keydown", onKeyDown);
+    return () => document.removeEventListener("keydown", onKeyDown);
+  }, []);
+
+  const navContent = (
+    <>
+      <div className="flex h-14 items-center justify-between border-b border-neutral-200 px-4">
         <span className="text-sm font-semibold text-neutral-900">KanzlAI</span>
+        <button
+          onClick={() => setMobileOpen(false)}
+          className="rounded-md p-1 text-neutral-400 transition-colors hover:bg-neutral-100 hover:text-neutral-600 lg:hidden"
+          aria-label="Menü schließen"
+        >
+          <X className="h-4 w-4" />
+        </button>
       </div>
       <nav className="flex-1 space-y-0.5 p-2">
         {navigation.map((item) => {
-          const isActive = pathname.startsWith(item.href);
+          const isActive =
+            pathname === item.href || pathname.startsWith(item.href + "/");
           return (
             <Link
               key={item.href}
               href={item.href}
-              className={`flex items-center gap-2.5 rounded-md px-2.5 py-1.5 text-sm transition-colors ${
+              className={`flex items-center gap-2.5 rounded-md px-2.5 py-2 text-sm transition-colors ${
                 isActive
                   ? "bg-neutral-100 font-medium text-neutral-900"
                   : "text-neutral-600 hover:bg-neutral-50 hover:text-neutral-900"
@@ -47,6 +73,39 @@ export function Sidebar() {
           );
         })}
       </nav>
-    </aside>
+    </>
+  );
+
+  return (
+    <>
+      {/* Mobile hamburger button */}
+      <button
+        onClick={() => setMobileOpen(true)}
+        className="fixed left-3 top-3.5 z-40 rounded-md bg-white p-1.5 shadow-sm ring-1 ring-neutral-200 transition-colors hover:bg-neutral-50 lg:hidden"
+        aria-label="Menü öffnen"
+      >
+        <Menu className="h-5 w-5 text-neutral-700" />
+      </button>
+
+      {/* Mobile overlay */}
+      {mobileOpen && (
+        <div
+          className="fixed inset-0 z-40 bg-black/20 backdrop-blur-sm lg:hidden"
+          onClick={() => setMobileOpen(false)}
+        />
+      )}
+
+      {/* Mobile sidebar */}
+      {mobileOpen && (
+        <aside className="animate-slide-in-left fixed inset-y-0 left-0 z-50 flex w-56 flex-col border-r border-neutral-200 bg-white shadow-lg lg:hidden">
+          {navContent}
+        </aside>
+      )}
+
+      {/* Desktop sidebar */}
+      <aside className="hidden h-full w-56 flex-col border-r border-neutral-200 bg-white lg:flex">
+        {navContent}
+      </aside>
+    </>
   );
 }