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/DocumentUpload.tsx

@@ -0,0 +1,144 @@
+"use client";
+
+import { useCallback, useState } from "react";
+import { useDropzone } from "react-dropzone";
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+import { Upload, FileText, X, Loader2 } from "lucide-react";
+import { toast } from "sonner";
+import { api } from "@/lib/api";
+import type { Document } from "@/lib/types";
+
+interface DocumentUploadProps {
+  caseId: string;
+}
+
+export function DocumentUpload({ caseId }: DocumentUploadProps) {
+  const [files, setFiles] = useState<File[]>([]);
+  const queryClient = useQueryClient();
+
+  const uploadMutation = useMutation({
+    mutationFn: async (file: File) => {
+      const formData = new FormData();
+      formData.append("file", file);
+      formData.append("title", file.name);
+      return api.postFormData<Document>(`/cases/${caseId}/documents`, formData);
+    },
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["case-documents", caseId] });
+      queryClient.invalidateQueries({ queryKey: ["case", caseId] });
+    },
+  });
+
+  const onDrop = useCallback((acceptedFiles: File[]) => {
+    setFiles((prev) => [...prev, ...acceptedFiles]);
+  }, []);
+
+  const { getRootProps, getInputProps, isDragActive } = useDropzone({
+    onDrop,
+    disabled: uploadMutation.isPending,
+  });
+
+  function removeFile(index: number) {
+    setFiles((prev) => prev.filter((_, i) => i !== index));
+  }
+
+  async function handleUpload() {
+    if (files.length === 0) return;
+
+    let successCount = 0;
+    for (const file of files) {
+      try {
+        await uploadMutation.mutateAsync(file);
+        successCount++;
+      } catch (err) {
+        const msg =
+          err && typeof err === "object" && "error" in err
+            ? (err as { error: string }).error
+            : file.name;
+        toast.error(`Fehler beim Hochladen: ${msg}`);
+      }
+    }
+
+    if (successCount > 0) {
+      toast.success(
+        successCount === 1
+          ? "Dokument hochgeladen"
+          : `${successCount} Dokumente hochgeladen`,
+      );
+      setFiles([]);
+    }
+  }
+
+  return (
+    <div className="space-y-3">
+      <div
+        {...getRootProps()}
+        className={`cursor-pointer rounded-md border-2 border-dashed px-6 py-6 text-center transition-colors ${
+          isDragActive
+            ? "border-neutral-500 bg-neutral-50"
+            : "border-neutral-300 hover:border-neutral-400"
+        } ${uploadMutation.isPending ? "pointer-events-none opacity-50" : ""}`}
+      >
+        <input {...getInputProps()} />
+        <Upload className="mx-auto h-6 w-6 text-neutral-400" />
+        <p className="mt-2 text-sm text-neutral-600">
+          Dateien hierher ziehen oder{" "}
+          <span className="font-medium text-neutral-900">durchsuchen</span>
+        </p>
+        <p className="mt-1 text-xs text-neutral-400">Max. 50 MB pro Datei</p>
+      </div>
+
+      {files.length > 0 && (
+        <div className="space-y-2">
+          {files.map((file, i) => (
+            <div
+              key={`${file.name}-${i}`}
+              className="flex items-center gap-3 rounded-md border border-neutral-200 bg-neutral-50 px-3 py-2"
+            >
+              <FileText className="h-4 w-4 shrink-0 text-neutral-500" />
+              <div className="min-w-0 flex-1">
+                <p className="truncate text-sm text-neutral-900">{file.name}</p>
+                <p className="text-xs text-neutral-400">
+                  {formatFileSize(file.size)}
+                </p>
+              </div>
+              <button
+                type="button"
+                onClick={() => removeFile(i)}
+                disabled={uploadMutation.isPending}
+                className="rounded p-1 text-neutral-400 hover:bg-neutral-200 hover:text-neutral-600"
+              >
+                <X className="h-3.5 w-3.5" />
+              </button>
+            </div>
+          ))}
+
+          <button
+            type="button"
+            onClick={handleUpload}
+            disabled={uploadMutation.isPending}
+            className="inline-flex items-center gap-2 rounded-md bg-neutral-900 px-3 py-1.5 text-sm font-medium text-white hover:bg-neutral-800 disabled:opacity-50"
+          >
+            {uploadMutation.isPending ? (
+              <>
+                <Loader2 className="h-3.5 w-3.5 animate-spin" />
+                Hochladen...
+              </>
+            ) : (
+              <>
+                <Upload className="h-3.5 w-3.5" />
+                {files.length === 1 ? "Hochladen" : `${files.length} Dateien hochladen`}
+              </>
+            )}
+          </button>
+        </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`;
+}