feat(account): danger zone, restore import, larger API key field on phones

- Danger zone in settings: delete all own data or the account, both with
  password confirmation; the last server admin cannot delete the account
- DELETE /user/data and /user/account; PurgeUserData removes everything a
  user owns in one transaction (others' tasks in shared lists stay theirs),
  also used by admin user deletion
- MindSift import: 'Hinzufügen' or 'Wiederherstellen' (mode=restore, wipes
  own data only after the file is fully validated, confirm checkbox)
- API key name field and button 44 px on phones (flex-1 collapsed them)

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

frontend/src/features/settings/EcosystemView.tsx

@@ -115,11 +115,11 @@ export const EcosystemView: React.FC<EcosystemViewProps> = ({ onIngested }) => {
             value={newKeyName}
             onChange={(e) => setNewKeyName(e.target.value)}
             placeholder="Bezeichnung (z. B. Home Assistant, Proxmox Backup, Rauchmelder)…"
-            className="flex-1 h-9 px-3 input-field text-sm"
+            className="w-full sm:flex-1 min-w-0 h-11 sm:h-9 px-3 input-field text-base sm:text-sm"
           />
           <button
             type="submit"
-            className="btn-primary h-9 px-4 text-xs sm:text-sm"
+            className="btn-primary h-11 sm:h-9 px-4 text-sm flex items-center justify-center gap-1.5"
           >
             <Plus className="w-4 h-4" />
             <span>API-Key erzeugen</span>