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>

Doku/api/rest.md

@@ -31,6 +31,7 @@ Basis: `/api/v1`. Antworten sind JSON; Fehler haben die Form `{"error": "Text"}`
 | GET | `/stats` | Zähler fürs Dashboard |
 | GET | `/search?q=` | Suche in eigenen Gedanken und Notizen |
 | GET | `/export` | Datenexport als JSON |
+| GET | `/export/markdown` | Obsidian-kompatibles ZIP (`mindsift-vault-DATUM.zip`), direkt gestreamt: Notizen als `.md` mit YAML-Frontmatter in der Ordnerstruktur des Nutzers (inkl. geteilter), `Aufgaben/<Liste>.md` im Format des Tasks-Plugins (`📅`, `⏫`/`🔺`), `Listen/<Liste>.md` |
 | GET/POST | `/themes` | Eigene CSS-Themes; anlegen oder mit `id` ändern (`name`, `css`, max. 200 KB) |
 | DELETE | `/themes/{id}` | Eigenes Theme löschen |
 | GET/POST/DELETE | `/keys`, `/keys/{id}` | API-Keys verwalten (Klartext nur bei Erstellung) |