feat: add fission package download

frontend/src/app/fission/[id]/notes/page.tsx

@@ -56,6 +56,7 @@ export default function FissionNotesPage() {
   const fissionId = Number(params?.id);
   const [data, setData] = useState<NotesResponse | null>(null);
   const [error, setError] = useState<string | null>(null);
+  const [downloading, setDownloading] = useState(false);
 
   const fetchNotes = useCallback(async () => {
     setError(null);
@@ -86,6 +87,27 @@ export default function FissionNotesPage() {
     setTimeout(tick, 8000);
   }, [fissionId, fetchNotes]);
 
+  const downloadPackage = useCallback(async () => {
+    if (!fissionId || downloading) return;
+    setDownloading(true);
+    setError(null);
+    try {
+      const blob = await api.getBlob(`/api/v1/fission/${fissionId}/download`);
+      const url = URL.createObjectURL(blob);
+      const a = document.createElement('a');
+      a.href = url;
+      a.download = `fission-${fissionId}.zip`;
+      document.body.appendChild(a);
+      a.click();
+      document.body.removeChild(a);
+      URL.revokeObjectURL(url);
+    } catch {
+      setError('下载裂变包失败，请稍后重试');
+    } finally {
+      setDownloading(false);
+    }
+  }, [fissionId, downloading]);
+
   return (
     <div className="flex flex-col gap-6 p-6 max-w-4xl">
       <div className="flex items-center justify-between">
@@ -97,9 +119,19 @@ export default function FissionNotesPage() {
             一个爆款裂变出的多套差异化完整笔记，可分发给不同达人。
           </p>
         </div>
-        <Link href="/fission" className="text-sm text-text-secondary underline">
-          返回裂变
-        </Link>
+        <div className="flex items-center gap-3">
+          <button
+            type="button"
+            onClick={downloadPackage}
+            disabled={!data || data.notes.length === 0 || downloading}
+            className="rounded-lg bg-brand-orange px-4 py-2 text-sm font-medium text-white disabled:cursor-not-allowed disabled:opacity-50"
+          >
+            {downloading ? '下载中…' : '下载裂变包'}
+          </button>
+          <Link href="/fission" className="text-sm text-text-secondary underline">
+            返回裂变
+          </Link>
+        </div>
       </div>
 
       {error && (