feat: add document management frontend (Phase 2N)

- DocumentUpload: dropzone with multi-file support, upload via
  POST /api/cases/{id}/documents, progress feedback with toast
- DocumentList: type badges, file size, upload date, download links,
  delete with inline confirmation
- Integrated as Dokumente tab in case detail page with count badge
- Eagerly fetches document count for tab badge display

frontend/src/components/documents/DocumentList.tsx

@@ -0,0 +1,144 @@
+"use client";
+
+import { useState } from "react";
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+import { FileText, Download, Trash2, Loader2 } from "lucide-react";
+import { format } from "date-fns";
+import { de } from "date-fns/locale";
+import { toast } from "sonner";
+import { api } from "@/lib/api";
+import type { Document } from "@/lib/types";
+
+const DOC_TYPE_BADGE: Record<string, string> = {
+  schriftsatz: "bg-blue-50 text-blue-700",
+  beschluss: "bg-violet-50 text-violet-700",
+  urteil: "bg-emerald-50 text-emerald-700",
+  gutachten: "bg-amber-50 text-amber-700",
+  vertrag: "bg-cyan-50 text-cyan-700",
+  korrespondenz: "bg-neutral-100 text-neutral-600",
+};
+
+interface DocumentListProps {
+  documents: Document[];
+  caseId: string;
+}
+
+export function DocumentList({ documents, caseId }: DocumentListProps) {
+  const [deleteId, setDeleteId] = useState<string | null>(null);
+  const queryClient = useQueryClient();
+
+  const deleteMutation = useMutation({
+    mutationFn: (docId: string) => api.delete(`/documents/${docId}`),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["case-documents", caseId] });
+      queryClient.invalidateQueries({ queryKey: ["case", caseId] });
+      toast.success("Dokument geloescht");
+      setDeleteId(null);
+    },
+    onError: (err) => {
+      const msg =
+        err && typeof err === "object" && "error" in err
+          ? (err as { error: string }).error
+          : "Unbekannter Fehler";
+      toast.error(`Fehler beim Loeschen: ${msg}`);
+      setDeleteId(null);
+    },
+  });
+
+  if (documents.length === 0) {
+    return (
+      <p className="py-8 text-center text-sm text-neutral-400">
+        Keine Dokumente vorhanden.
+      </p>
+    );
+  }
+
+  return (
+    <div className="space-y-2">
+      {documents.map((doc) => (
+        <div
+          key={doc.id}
+          className="flex items-center justify-between rounded-md border border-neutral-200 bg-white px-4 py-3"
+        >
+          <div className="flex items-center gap-3 min-w-0">
+            <FileText className="h-4 w-4 shrink-0 text-neutral-400" />
+            <div className="min-w-0">
+              <p className="truncate text-sm font-medium text-neutral-900">
+                {doc.title}
+              </p>
+              <div className="mt-0.5 flex flex-wrap items-center gap-2 text-xs text-neutral-400">
+                {doc.doc_type && (
+                  <span
+                    className={`rounded-full px-2 py-0.5 text-xs font-medium ${
+                      DOC_TYPE_BADGE[doc.doc_type.toLowerCase()] ??
+                      "bg-neutral-100 text-neutral-600"
+                    }`}
+                  >
+                    {doc.doc_type}
+                  </span>
+                )}
+                {doc.file_size != null && (
+                  <span>{formatFileSize(doc.file_size)}</span>
+                )}
+                <span>
+                  {format(new Date(doc.created_at), "d. MMM yyyy", {
+                    locale: de,
+                  })}
+                </span>
+              </div>
+            </div>
+          </div>
+
+          <div className="flex items-center gap-1 shrink-0 ml-3">
+            <a
+              href={`/api/documents/${doc.id}`}
+              className="rounded p-1.5 text-neutral-400 hover:bg-neutral-100 hover:text-neutral-600"
+              title="Herunterladen"
+            >
+              <Download className="h-4 w-4" />
+            </a>
+
+            {deleteId === doc.id ? (
+              <div className="flex items-center gap-1">
+                <button
+                  type="button"
+                  onClick={() => deleteMutation.mutate(doc.id)}
+                  disabled={deleteMutation.isPending}
+                  className="rounded px-2 py-1 text-xs font-medium text-red-600 hover:bg-red-50"
+                >
+                  {deleteMutation.isPending ? (
+                    <Loader2 className="h-3.5 w-3.5 animate-spin" />
+                  ) : (
+                    "Loeschen"
+                  )}
+                </button>
+                <button
+                  type="button"
+                  onClick={() => setDeleteId(null)}
+                  className="rounded px-2 py-1 text-xs text-neutral-500 hover:bg-neutral-100"
+                >
+                  Abbrechen
+                </button>
+              </div>
+            ) : (
+              <button
+                type="button"
+                onClick={() => setDeleteId(doc.id)}
+                className="rounded p-1.5 text-neutral-400 hover:bg-neutral-100 hover:text-red-500"
+                title="Loeschen"
+              >
+                <Trash2 className="h-4 w-4" />
+              </button>
+            )}
+          </div>
+        </div>
+      ))}
+    </div>
+  );
+}
+
+function formatFileSize(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(0)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}