feat(export): Obsidian-compatible Markdown vault as streamed ZIP

- GET /export/markdown streams the workspace as ZIP (no temp files):
  notes as .md with YAML front matter in the user's folder structure
  (shared notes and mounted folders included, empty folders kept)
- Clean Markdown: top-level headings/dividers/callouts/code as real blocks
  (their children move up a level), lists with tabs, todos as - [ ]
- Aufgaben/<list>.md in Obsidian Tasks format (due date, priority,
  subtasks, notes), Listen/<list>.md for shopping and other lists
- Safe, unique file names; download button in settings (mobile-safe)

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

frontend/src/lib/api.ts

@@ -1074,6 +1074,17 @@ export const api = {
   async exportAll(): Promise<any> {
     return request<any>('/api/v1/export');
   },
+
+  // 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();
+  },
 };
 
 export function getOfflineQueue(): Thought[] {