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/Header.tsx

@@ -25,16 +25,19 @@ export function Header() {
 
   return (
     <header className="flex h-14 items-center justify-between border-b border-neutral-200 bg-white px-4">
-      <div />
-      <div className="flex items-center gap-3">
+      {/* Spacer for mobile hamburger */}
+      <div className="w-8 lg:w-0" />
+      <div className="flex items-center gap-2 sm:gap-3">
         <TenantSwitcher />
         {email && (
-          <span className="text-sm text-neutral-500">{email}</span>
+          <span className="hidden text-sm text-neutral-500 sm:inline">
+            {email}
+          </span>
         )}
         <button
           onClick={handleLogout}
           title="Abmelden"
-          className="rounded-md p-1.5 text-neutral-400 hover:bg-neutral-100 hover:text-neutral-600"
+          className="rounded-md p-1.5 text-neutral-400 transition-colors hover:bg-neutral-100 hover:text-neutral-600"
         >
           <LogOut className="h-4 w-4" />
         </button>