refactor(useForm): improve defaults update logic

Refactor the logic for updating form defaults to handle single and multiple field updates more clearly. This change improves code readability and ensures that defaults are always cloned before modification.

Author: brodienguyen

packages/svelte5/src/useForm.svelte.ts

@@ -127,11 +127,16 @@ export default function useForm<TForm extends FormDataType<TForm>>(
 
       if (typeof fieldOrFields === 'undefined') {
         defaults = cloneDeep(getData())
+      } else if (typeof fieldOrFields === 'string') {
+        // Update single field
+        const newDefaults = cloneDeep(defaults)
+        set(newDefaults, fieldOrFields, maybeValue)
+        defaults = newDefaults
       } else {
-        defaults =
-          typeof fieldOrFields === 'string'
-            ? set(cloneDeep(defaults), fieldOrFields, maybeValue)
-            : Object.assign(cloneDeep(defaults), fieldOrFields)
+        // Update multiple fields
+        const newDefaults = cloneDeep(defaults)
+        Object.assign(newDefaults, fieldOrFields)
+        defaults = newDefaults
       }
 
       return form