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/app/(app)/cases/page.tsx

@@ -5,6 +5,7 @@ import { api } from "@/lib/api";
 import type { Case } from "@/lib/types";
 import Link from "next/link";
 import { useSearchParams, useRouter } from "next/navigation";
+import { Breadcrumb } from "@/components/layout/Breadcrumb";
 import { Plus, Search, FolderOpen } from "lucide-react";
 import { useState } from "react";
 import { SkeletonTable } from "@/components/ui/Skeleton";
@@ -72,6 +73,12 @@ export default function CasesPage() {
 
   return (
     <div className="animate-fade-in">
+      <Breadcrumb
+        items={[
+          { label: "Dashboard", href: "/dashboard" },
+          { label: "Akten" },
+        ]}
+      />
       <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">Akten</h1>