feat(import): restore MindSift data and import Obsidian vaults

- Two separate paths: POST /import/mindsift (JSON backup or the Markdown
  archive) and POST /import/obsidian (vault ZIP into a folder named after it)
- Everything is planned and validated first; ?preview=1 returns what would
  be created and what is skipped (with reason), nothing is written
- MindSift data round-trips: folders, note structure, task lists with
  subtasks/due dates/priorities/notes, lists with quantity/category/done,
  triage thoughts from the JSON backup
- Markdown to outliner: nested lists, heading sections, callouts, code,
  paragraphs; front matter tags become hashtags; .obsidian/attachments skipped
- Limits against zip bombs and path tricks (100 MB upload, 10 MB per note,
  500 MB unpacked, 20k files, no .. escapes)
- Settings: two import cards with drag & drop, preview, confirm, result;
  touch-sized buttons; mobile download/upload verified

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

frontend/src/features/settings/UserSettingsView.tsx

@@ -27,6 +27,7 @@ import themeTemplateCss from '../../themes/theme-template.css?raw';
 import { UserAvatar, resizeImageToDataUrl } from '../../components/UserAvatar';
 import { LegalLinks } from '../../components/LegalLinks';
 import { AboutModal } from '../../components/AboutModal';
+import { ImportCard } from './ImportCard';
 
 interface UserSettingsViewProps {
   currentUser: User;
@@ -1273,6 +1274,42 @@ export const UserSettingsView: React.FC<UserSettingsViewProps> = ({
                   <span>{exportingVault ? 'Wird erstellt…' : 'Markdown-Archiv herunterladen'}</span>
                 </button>
               </div>
+
+              {/* Import: two separate paths, never guessed from the file */}
+              <div className="pt-4 border-t border-border space-y-3">
+                <div>
+                  <h3 className="font-bold text-main">Daten importieren</h3>
+                  <p className="text-muted text-[11px]">
+                    Ein Import legt alles neu an und ändert nichts Bestehendes. Vor dem Import siehst du, was angelegt wird.
+                  </p>
+                </div>
+                <div className="grid grid-cols-1 lg:grid-cols-2 gap-3">
+                  <ImportCard
+                    source="mindsift"
+                    title="MindSift-Daten"
+                    accept=".json,.zip,application/json,application/zip"
+                    description={
+                      <>
+                        Eigenes Backup zurückspielen: JSON-Backup (<code className="font-mono">.json</code>) oder Markdown-Archiv
+                        (<code className="font-mono">.zip</code>). Notizen kommen in ihre Ordner, Aufgabenlisten mit Unteraufgaben,
+                        Fälligkeiten und Prioritäten sowie Listen werden wiederhergestellt.
+                      </>
+                    }
+                  />
+                  <ImportCard
+                    source="obsidian"
+                    title="Obsidian-Vault"
+                    accept=".zip,application/zip"
+                    description={
+                      <>
+                        Vault als <code className="font-mono">.zip</code>. Alle Notizen landen in einem eigenen Ordner mit dem Namen
+                        des Vaults, Unterordner bleiben erhalten. Checkboxen bleiben Checkboxen in der Notiz, Frontmatter-Tags werden zu
+                        #Hashtags. Bilder und andere Anhänge werden übersprungen.
+                      </>
+                    }
+                  />
+                </div>
+              </div>
             </div>
           )}
         </div>