feat(12.2-02): add the datepicker field with server-side bounds and date list columns

- type: datepicker compiles the D-20 keys; format maps to displayFormat with WinterCMS's momentFormat table
- boot fails when the mode does not match the column's Go type (time.Time, lagoon.Date, lagoon.TimeOfDay)
- datepicker is a writable scalar field; minDate and maxDate are rechecked on save
- columns.yaml accepts type: date and type: time; Scanner/Valuer structs are columns, not relations
- conformance fixture carries date and datetime fields; README, forms and lists docs

modules/cabana/settings.go

@@ -84,6 +84,11 @@ func compileSetting(pluginID string, item pact.SettingsItem, fsys fs.FS) (*Compi
 		if _, ok := fillable[field.Name]; !ok {
 			return nil, fmt.Errorf("cabana: setting %s field %s is not fillable", item.Code, field.Name)
 		}
+		if field.Type == "datepicker" {
+			if err := checkDateType(model, field); err != nil {
+				return nil, fmt.Errorf("cabana: setting %s: %w", item.Code, err)
+			}
+		}
 		writable = append(writable, WritableField{Name: field.Name, FillKey: field.Name})
 	}
 	return &CompiledSetting{PluginID: pluginID, Item: item, Form: form, Writable: writable}, nil