Update display preferences

Partial upsert. weekStart is monday or sunday; roundingMinutes one of 0, 5, 6, 10, 15, 30 (display-level rounding); currency a 3-letter ISO code. Preferences are per-user, not per-org.

curl -X PUT "https://example.com/v1/preferences" \  -H "Content-Type: application/json" \  -d '{}'

{  "preferences": {    "weekStart": "monday",    "roundingMinutes": 0,    "currency": "string"  }}