Update user settings and preferences
| Field | Type | Description |
|---|---|---|
delivery_formats | array | Preferred output formats in order |
default_quality | integer | Default quality (1-100) |
default_fit_mode | string | Default fit mode |
default_max_width | integer | Default max width (null to clear) |
default_max_height | integer | Default max height (null to clear) |
theme | string | UI theme preference |
language | string | UI language preference |