feat(backup): download the full backup as ZIP

GET /export?format=zip streams the JSON backup compressed (plain JSON stays
the API default). The MindSift import recognises the backup ZIP next to the
plain JSON and the Markdown archive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -135,6 +135,14 @@ export const setCurrentSharingMode = (mode: SharingMode) => {
 };
 export const isSharingDisabled = () => currentSharingMode === 'private';
 
+// Authenticated binary download (archives); no JSON parsing and no request timeout
+async function downloadBlob(path: string): Promise<Blob> {
+  const token = getToken();
+  const res = await fetch(path, { headers: token ? { Authorization: `Bearer ${token}` } : {}, cache: 'no-store' });
+  if (!res.ok) throw new Error(`Download fehlgeschlagen (HTTP ${res.status})`);
+  return res.blob();
+}
+
 export const api = {
   // --- AUTH, FIRST-BOOT SETUP & SETTINGS ---
   async getAuthStatus(): Promise<AuthStatus> {
@@ -1093,15 +1101,14 @@ export const api = {
     return data as ImportSummary;
   },
 
+  // Full backup (all data as JSON) packed into a ZIP; restorable via importData('mindsift', …)
+  async exportBackupZip(): Promise<Blob> {
+    return downloadBlob('/api/v1/export?format=zip');
+  },
+
   // Obsidian-compatible ZIP of the workspace; binary and possibly large, so no JSON helper and no timeout
   async exportMarkdownVault(): Promise<Blob> {
-    const token = getToken();
-    const res = await fetch('/api/v1/export/markdown', {
-      headers: token ? { Authorization: `Bearer ${token}` } : {},
-      cache: 'no-store',
-    });
-    if (!res.ok) throw new Error(`Export fehlgeschlagen (HTTP ${res.status})`);
-    return res.blob();
+    return downloadBlob('/api/v1/export/markdown');
   },
 };