feat: Phase B — interactive dashboard, breadcrumbs, clickable navigation

- Breadcrumb component: reusable nav with items array (label+href)
- DeadlineTrafficLights: buttons → Links to /fristen?status={filter}
- CaseOverviewGrid: static metrics → clickable Links to /cases?status={filter}
- UpcomingTimeline: items → clickable Links to /fristen/{id} or /termine/{id}
  with case number links and hover chevron
- QuickActions: swap CalDAV Sync for "Neuer Termin" → /termine/neu,
  fix "Frist eintragen" → /fristen/neu
- AISummaryCard: add RefreshCw button with spinning animation
- RecentActivityList: new component showing recent case events
- DeadlineList: accept initialStatus prop, add this_week/ok filters
- fristen/page.tsx: read searchParams.status for initial filter
- Add breadcrumbs to dashboard, fristen, cases, termine pages
- Add RecentActivity type, update DashboardData type

frontend/src/components/layout/Breadcrumb.tsx

@@ -0,0 +1,38 @@
+import Link from "next/link";
+import { ChevronRight } from "lucide-react";
+
+export interface BreadcrumbItem {
+  label: string;
+  href?: string;
+}
+
+interface Props {
+  items: BreadcrumbItem[];
+}
+
+export function Breadcrumb({ items }: Props) {
+  return (
+    <nav aria-label="Breadcrumb" className="mb-4 flex items-center gap-1 text-sm text-neutral-500">
+      {items.map((item, i) => {
+        const isLast = i === items.length - 1;
+        return (
+          <span key={i} className="flex items-center gap-1">
+            {i > 0 && <ChevronRight className="h-3.5 w-3.5 text-neutral-300" />}
+            {isLast || !item.href ? (
+              <span className={isLast ? "font-medium text-neutral-900" : ""}>
+                {item.label}
+              </span>
+            ) : (
+              <Link
+                href={item.href}
+                className="transition-colors hover:text-neutral-900"
+              >
+                {item.label}
+              </Link>
+            )}
+          </span>
+        );
+      })}
+    </nav>
+  );
+}